Add loading state when photos dropped into app

[gavr123456789]

When I add 100-200 photos Curtail always freezes for like 30 sec, app stop responding, and after that it starts processing. I think it generating thumbnails on this time. It would be better UX experience to show some loading with % or just spinner

[Huluti]

I tried to disable thumbnails generation to see if loading time comes from that and it the same for me.
I think that its the Gtk.FileDialog that becomes longer with many files.

Will try to see how we can handle this.