Allow to load a background image via drag and drop

[passiomatic]

Currently isn't possible to drop a background image into an element, because this will create a new image node. Hence you have to specify the image URL in the background -> image inspector panel.

The idea is to remove the editable image URL field and to allow to drop the image in the background inspector panel. The operation will trigger the usual upload progress bar, and at the end it will assign the uploaded image to the selected element background.

We can put back the image URL as a readonly field with a "copy" button aside (just like the regular image node 'info' panel).