cc-background-panel: Implement drag and drop for adding new background images
The current background panel only allows to add a new backround image by using a button. It seems very initutive to be able to add new background images by dragging them into the view.
Implement a simple way to add new background images using GtkDropTarget. This can be further refined by displaying a overlay indicating a file can be dropped onto the background panel.
Fixes: #958