QmlDesigner: Improve Welcome screen scollbar

- scroll bar only visible when needed
- change color when active
- remove useless stuff
- remove 3 warnings

Change-Id: I0fbab4ca574c060f85adb0d5b6b54381337a7eae
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/plugins/studiowelcome/qml/welcomepage/CustomScrollBar.qml

@@ -35,23 +35,20 @@ Controls.ScrollBar {
     implicitHeight: Math.max(implicitBackgroundHeight + topInset + bottomInset,
                              implicitContentHeight + topPadding + bottomPadding)
 
-    padding: control.interactive ? 1 : 2
-    visible: control.policy !== Controls.ScrollBar.AlwaysOff
-    minimumSize: orientation == Qt.Horizontal ? height / width : width / height
+    padding: active ? 1 : 2
+    visible: orientation === Qt.Horizontal ? contentWidth > width : contentHeight > height
+    minimumSize: orientation === Qt.Horizontal ? height / width : width / height
 
     contentItem: Rectangle {
         implicitWidth: 13
         implicitHeight: 13
-
-        color: Constants.textDefaultColor
+        color: active ? Constants.textHoverColor : Constants.textDefaultColor
     }
 
     background: Rectangle {
         implicitWidth: 16
         implicitHeight: 16
-        color: "#2d2e30"
-        visible: control.interactive
+        color: "#3b3c3d"
+        visible: active
     }
 }
-
-

src/plugins/studiowelcome/qml/welcomepage/CustomScrollView.qml

@@ -33,10 +33,5 @@ Controls.ScrollView {
            x: control.mirrored ? 0 : control.width - width
            y: control.topPadding
            height: control.availableHeight
-           active: control.ScrollBar.horizontal.active
-       }
-
-       Controls.ScrollBar.horizontal: CustomScrollBar {
-           visible: false
        }
 }