Slightly improve the layout of the login/logout/lock screens

Currently on the login/logout/lock screens, there is almost no spacing between
the avatar and username, which makes the user' delegate looks too tight.This MR 
fixes this issue.

lookandfeel/contents/components/UserDelegate.qml

@@ -33,8 +33,6 @@ Item {
 
     property real faceSize: PlasmaCore.Units.gridUnit * 7
 
-    implicitHeight: imageSource.height + usernameDelegate.height
-    implicitWidth: Math.max(imageSource.width, usernameDelegate.width)
     opacity: isCurrent ? 1.0 : 0.5
 
     Behavior on opacity {
@@ -150,12 +148,13 @@ Item {
         id: usernameDelegate
 
         anchors.top: imageSource.bottom
+        anchors.topMargin: PlasmaCore.Units.gridUnit
         anchors.horizontalCenter: parent.horizontalCenter
 
         // Make it bigger than other fonts to match the scale of the avatar better
         font.pointSize: wrapper.fontSize + 4
 
-        width: constrainText ? imageSource.width : implicitWidth
+        width: constrainText ? parent.width : implicitWidth
         text: wrapper.name
         style: softwareRendering ? Text.Outline : Text.Normal
         styleColor: softwareRendering ? PlasmaCore.ColorScope.backgroundColor : "transparent" //no outline, doesn't matter

lookandfeel/contents/components/UserList.qml

@@ -12,7 +12,7 @@ ListView {
     id: view
     readonly property string selectedUser: currentItem ? currentItem.userName : ""
     readonly property int userItemWidth: PlasmaCore.Units.gridUnit * 8
-    readonly property int userItemHeight: PlasmaCore.Units.gridUnit * 8
+    readonly property int userItemHeight: PlasmaCore.Units.gridUnit * 9
     readonly property bool constrainText: count > 1
     property int fontSize: PlasmaCore.Theme.defaultFont.pointSize + 2
 

lookandfeel/contents/logout/Logout.qml

@@ -40,7 +40,7 @@ PlasmaCore.ColorScope {
     function hibernateRequested() {
         root.suspendRequested(4);
     }
- 
+
     property real timeout: 30
     property real remainingTime: root.timeout
     property var currentAction: {
@@ -104,6 +104,8 @@ PlasmaCore.ColorScope {
         onClicked: root.cancelRequested()
     }
     UserDelegate {
+        width: PlasmaCore.Units.gridUnit * 8
+        height: PlasmaCore.Units.gridUnit * 9
         anchors {
             horizontalCenter: parent.horizontalCenter
             bottom: parent.verticalCenter