Improve correctness of generated xwidget events

Improve correctness of generated xwidget events

* src/xwidget.c (xw_maybe_synthesize_crossing): Add new
parameters for controlling the crossing mode.  Also improve the
accuracy of generated crossing events when the mouse pointer
moves outside the toplevel from an inferior of it.  All callers
changed.
(xw_notify_virtual_upwards_until):
(xw_notify_virtual_downwards_until): New parameters for crossing
mode.  All callers changed.