[Notifications] Split draggable thumbnail area into its own component

I'm planning to use it in job notifications, too.
This should also make it easier to migrate away from a custom drag implementation to using Qt's if we were to do that.

Differential Revision: https://phabricator.kde.org/D25751

applets/notifications/package/contents/ui/DraggableFileArea.qml

@@ -0,0 +1,71 @@
+/*
+ *   Copyright 2016,2019 Kai Uwe Broulik <kde@privat.broulik.de>
+ *
+ *   This program is free software; you can redistribute it and/or modify
+ *   it under the terms of the GNU Library General Public License as
+ *   published by the Free Software Foundation; either version 2, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *   GNU Library General Public License for more details
+ *
+ *   You should have received a copy of the GNU Library General Public
+ *   License along with this program; if not, write to the
+ *   Free Software Foundation, Inc.,
+ *   51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ */
+
+import QtQuick 2.8
+
+MouseArea {
+    id: area
+
+    signal activated
+    signal contextMenuRequested(int x, int y)
+
+    property Item dragParent
+    property url dragUrl
+    property var dragPixmap
+
+    readonly property bool dragging: plasmoid.nativeInterface.dragActive
+
+    property int _pressX: -1
+    property int _pressY: -1
+
+    preventStealing: true
+    cursorShape: pressed ? Qt.ClosedHandCursor : Qt.OpenHandCursor
+    acceptedButtons: Qt.LeftButton | Qt.RightButton
+
+    onClicked: {
+        if (mouse.button === Qt.LeftButton) {
+            area.activated();
+        }
+    }
+    onPressed: {
+        if (mouse.button === Qt.LeftButton) {
+            _pressX = mouse.x;
+            _pressY = mouse.y;
+        } else if (mouse.button === Qt.RightButton) {
+            area.contextMenuRequested(mouse.x, mouse.y);
+        }
+    }
+    onPositionChanged: {
+        if (_pressX !== -1 && _pressY !== -1 && plasmoid.nativeInterface.isDrag(_pressX, _pressY, mouse.x, mouse.y)) {
+            plasmoid.nativeInterface.startDrag(area.dragParent, area.dragUrl, area.dragPixmap);
+            _pressX = -1;
+            _pressY = -1;
+        }
+    }
+    onReleased: {
+        _pressX = -1;
+        _pressY = -1;
+    }
+    onContainsMouseChanged: {
+        if (!containsMouse) {
+            _pressX = -1;
+            _pressY = -1;
+        }
+    }
+}

applets/notifications/package/contents/ui/ThumbnailStrip.qml

@@ -29,7 +29,7 @@ import org.kde.kquickcontrolsaddons 2.0 as KQCAddons
 
 import org.kde.plasma.private.notifications 2.0 as Notifications
 
-MouseArea {
+DraggableFileArea {
     id: thumbnailArea
 
     // The protocol supports multiple URLs but so far it's only used to show
@@ -38,7 +38,6 @@ MouseArea {
     // for multiple files)
     property var urls
 
-    readonly property bool dragging: plasmoid.nativeInterface.dragActive
     readonly property alias menuOpen: fileMenu.visible
 
     property int _pressX: -1
@@ -52,48 +51,21 @@ MouseArea {
     signal openUrl(string url)
     signal fileActionInvoked
 
+    dragParent: previewPixmap
+    dragUrl: thumbnailer.url
+    dragPixmap: thumbnailer.pixmap
+
     implicitHeight: Math.max(menuButton.height + 2 * menuButton.anchors.topMargin,
                              Math.round(Math.min(width / 3, width / thumbnailer.ratio)))
                     + topPadding + bottomPadding
 
-    preventStealing: true
-    cursorShape: pressed ? Qt.ClosedHandCursor : Qt.OpenHandCursor
-    acceptedButtons: Qt.LeftButton | Qt.RightButton
-
-    onClicked: {
-        if (mouse.button === Qt.LeftButton) {
-            thumbnailArea.openUrl(thumbnailer.url)
-        }
-    }
-
-    onPressed: {
-        if (mouse.button === Qt.LeftButton) {
-            _pressX = mouse.x;
-            _pressY = mouse.y;
-        } else if (mouse.button === Qt.RightButton) {
-            // avoid menu button glowing if we didn't actually press it
-            menuButton.checked = false;
+    onActivated: thumbnailArea.openUrl(thumbnailer.url)
+    onContextMenuRequested: {
+        // avoid menu button glowing if we didn't actually press it
+        menuButton.checked = false;
 
-            fileMenu.visualParent = this;
-            fileMenu.open(mouse.x, mouse.y);
-        }
-    }
-    onPositionChanged: {
-        if (_pressX !== -1 && _pressY !== -1 && plasmoid.nativeInterface.isDrag(_pressX, _pressY, mouse.x, mouse.y)) {
-            plasmoid.nativeInterface.startDrag(previewPixmap, thumbnailer.url, thumbnailer.pixmap);
-            _pressX = -1;
-            _pressY = -1;
-        }
-    }
-    onReleased: {
-        _pressX = -1;
-        _pressY = -1;
-    }
-    onContainsMouseChanged: {
-        if (!containsMouse) {
-            _pressX = -1;
-            _pressY = -1;
-        }
+        fileMenu.visualParent = this;
+        fileMenu.open(x, y);
     }
 
     Notifications.FileMenu {