effects/overview: Prevent undesired dragging of desktops bar

Flickable by default allows swiping via click-and-drag or
scrolling with the mousewheel, which is weird when the content
is smaller than the available space.

Inhibit this behavior unless there's a need for scrolling.

src/effects/overview/qml/DesktopBar.qml

@@ -32,6 +32,7 @@ Item {
         topMargin: Math.max((height - contentHeight) / 2, 0)
         contentWidth: contentItem.childrenRect.width
         contentHeight: contentItem.childrenRect.height
+        interactive: contentWidth > width
         clip: true
         flickableDirection: Flickable.HorizontalFlick