Use WBackground and WFrame in dialogs.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Move winch_pending member from WDialog to WGroup.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Rename widget_redraw() to widget_draw().

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

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Rename mouse_h to mouse_handler_t.

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