Currently function shell_get_thumbnail_for_recent_info located in
src/shell-global.c is used to get thumbnails for recently used files.
However, it only works if you have a GtkRecentInfo object for the file,
even though the thumbnail generation code doesn't depend on it. This commit
renames the function to shell_get_thumbnail and makes it generic so that it
just takes two strings: a filename and a mimetype.
e478cc4c4e