WIP: general: Update thumbnails without refreshing desktop

Until now, a change in the content of a file just triggered a refresh of the attributes, and only if the file was a .desktop file. This means that every time a picture changed, a full desktop refresh was needed to update the thumbnail.

This patch changes this, and now desktop icons detects changes in the contents of an specific file, and triggers an update of the icon. Also, if it is a .desktop file, will also reload its content to ensure that any change will be honored.