Implement crossing event generation between windows for xwidgets on X

Implement crossing event generation between windows for xwidgets on X

* src/xwidget.c (enum xw_crossing_mode): New enum.
(xwidget_motion_notify):
(xwidget_motion_or_crossing): Synthesize crossing events if the
pointer moved between different windows.
(window_coords_from_toplevel):
(xw_maybe_synthesize_crossing): New functions.
(xwidget_init_view): Initialize new field to NULL.
(Fdelete_xwidget_view): Clear last crossing window.

* src/xwidget.h (struct xwidget_view): New field
`last_crossing_window'.