Adds drag support for the preview image in Save dialog

[igordsm]

Closes #29 . There are still some pending questions about the interaction:

1. should the dialog be closed after the dragging ends?
2. this only works if the drop target supports images. As such, it does not work when dropping the image into Files or browsers. This limitation also exists with copy to clipboard (issue Copy to Clipboard wont allow to paste as file (direct on files) #156 )

As always, thank you for the hard work in Elementary.

[danirabbit]

Thanks for working on this feature! I can confirm that this works with apps that accept pixbuf data like GIMP, for example.

I left a couple of inline comments with suggestions :)

We probably don't need to close the dialog if a drag is performed. Especially if the drop isn't successful this might lead to unexpected data loss

[igordsm]

@danrabbit Thank you for the suggestions. I have done as you said and re-requested a review.

[danirabbit]

Great! Thanks for implementing this feature!