Allow to "reload" ThunarFiles in a more granular way

Currently, e.g. thunar_thumbnailer_thumbnailer_finished calls thunar_file_reload on all files for which thumbnailing finished successfully.

However, it is actually not required to reload all aspects of the file when it got a thumbnail … it would be sufficient to use a new "thumbnail ready" signal in order to signal subscribers that they need to redraw the file image. (Ideally with the used thumbnail size as an argument)

That is just one use-case ... to be checked if there are others.