FolderNameFilter handles messages in multiple folders.

The FolderNameFilter used to look only at a message's primary folder (message.get_filename()) when deciding which tags to apply. This change allows the filter to look at all folders that a message exists in when applying the tags.

This was done by retrieving all filenames using message.get_filenames() and adding the appropriate processing to handle the list of file names.

This change would be especially useful for those using GMail to add tags to their messages.