A new widget to perform drag and drop operations, for example,
dropping ICS files to the GcalWindow. It allows to overlay
a child for visual feedback when a DnD operation is done.
either in the main window or in the import dialog,
depending on whether none could be imported, or just some
of them.
It is based on Loupe's DragOverlay widget.
Closes#1257
Diego Iván
dfc5a9b970