Implement xwidget passive grabs

Implement xwidget passive grabs

* src/xwidget.c (find_widget_at_pos): New parameters for
controlling whether to respect grabs.  All callers changed.
(window_coords_from_toplevel): Make work when the widget is
the toplevel.
(find_widget): Fix coding style.
(xwidget_button_1): Set and clear passive grabs if appropriate.
(xw_maybe_synthesize_crossing): Allow current_window to be NULL
if the mode is XW_CROSSING_LEFT.