New group API to select/unselect widget.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Modify update_cursor API to support groups.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Small refactoring of dialog size and position set/change.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Reimplement set widget size to support groups.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Reimplement dialog validation: use separate callback.

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