Add a means to easily update an image thumbnail

[humdinger]

This is hrev56098.

• Open an image that already has a thumbnail in an editor (ArtPaint, Wonderbrush...).
• Doodle a bit and save the image.

-> Tracker still shows the original thumbnail/icon

Can we have some automatism to re-generate a thumbnail? Or, if that's not possible, how would a software tell Tracker to re-generate the thumbnail of the image it is going to save?

Currently an "Identify" from the context menu only works if you have removed a Thumbnail attribute manually before. I suppose it should always re-generate thumbnails.