Add custom visual feedback for dropping an image

Fixes #59 (closed)

I diverged a bit from the design :

• Heavier blue, to avoid blandness on white backgrounds
• Solid 2px border to "unifiy" the shape. Makes it less confusing on some situations (e.g. a black screenshot with white margins. Without a solid part, it looks like three different bands)

I'm hiding floating buttons when the widget is added. The superposition was a bit odd.

Animation was not specified, I added a simple fade-in