Don't add application/x-rootwindow-drop to dragSourceSet targets (#2063)

When running under Wayland if we add application/x-rootwindow-drop to
dragSourceSet targets then we end up getting spammed with endless calls
onTitleDragDataGet() when we drag and drop title onto root window.

Also it seems like sidebar never added application/x-rootwindow-drop to
its dragSourceSet targets so I suppose this is fine.

This change along with #2062 fixes an issue described here: #1906