Use mc_event engine to handle focus/unfocus events.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
NOT FINISHED: new API to select/unselect widget.

TODO: add doxygen comments.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
NOT FINISHED: Modify update_cursor API to support groups.

TODO: add doxygen comments.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>