bracket_matched
Description:
iter is set to a valid iterator pointing to the matching bracket if state is 
        gtk_source_bracket_match_found.
 Otherwise iter is meaningless.
The signal is emitted only when the state changes, typically when the cursor moves.
A use-case for this signal is to show messages in a [class@Gtk.Statusbar].
Parameters:
| iter | if found, the location of the matching bracket. | 
| state | state of bracket matching. |