right now, in order to add a bookmark to the shortcuts pane of the GtkFileChooser default implementation you can select a path in the files pane and click on the 'Add' button or you can drag and drop the row from one pane to the other.

the inconsistency is in the validation of the paths: the 'Add' button becomes insensitive when selecting a file and returns sensitive when selecting a folder, suggesting that only folders can be bookmarks; if I drop a file on the shortcut pane, though, the bookmark is accepted.

also, now that the file chooser has three modes of operation (browse, search and recent files), we must keep consistency in all those cases. accepting only folders require validation inside the shortcuts pane drop function, because we might want to drag a file from the file chooser to another drop target. accepting all kind of paths would make the sensitivity checks go away.