Use introduced writable active in MnemonicData

AppMenu is using it's own way to remove mnemonics but then they still are being processed by Kirigami's MnemonicAttached.
This commit adapts AppMenu for use with writable active property to show mnemonics underlines based on own logic.

applets/appmenu/package/contents/ui/main.qml

@@ -25,6 +25,7 @@ import org.kde.kquickcontrolsaddons 2.0 // For KCMShell
 import org.kde.plasma.core 2.0 as PlasmaCore
 import org.kde.plasma.components 3.0 as PlasmaComponents3
 import org.kde.plasma.private.appmenu 1.0 as AppMenuPrivate
+import org.kde.kirigami 2.5 as Kirigami
 
 Item {
     id: root
@@ -118,19 +119,8 @@ Item {
 
                 Layout.fillWidth: root.vertical
                 Layout.fillHeight: !root.vertical
-                text: {
-                    var text = activeMenu;
-
-                    var alt = keystateSource.data.Alt;
-                    if (!alt || !alt.Pressed) {
-                        // StyleHelpers.removeMnemonics
-                        text = text.replace(/([^&]*)&(.)([^&]*)/g, function (match, p1, p2, p3) {
-                            return p1.concat(p2, p3);
-                        });
-                    }
-
-                    return text;
-                }
+                text: activeMenu
+                Kirigami.MnemonicData.active: keystateSource.data.Alt && keystateSource.data.Alt.Pressed
                 // fake highlighted
                 checkable: plasmoid.nativeInterface.currentIndex === index
                 checked: checkable