Allow xwidgets to accept motion and button events

Allow xwidgets to accept motion and button events

* src/xterm.c (handle_one_xevent): Pass through some events to
xwidgets.

* src/xwidget.c (synthesize_focus_in_event)
(find_suitable_pointer, xwidget_button_1)
(xwidget_button, xwidget_motion_or_crossing): New functions.
(xwidget_view_from_window): Draw offscreen window instead of the
widget.
(x_draw_xwidget_glyph_string): Set appropriate event masks.

* src/xwidget.h (xwidget_button, xwidget_motion_or_crossing): New
functions.