Use the real cursor beneath the pointer on xwidgets

Use the real cursor beneath the pointer on xwidgets

* src/xwidget.c (define_cursors): Make manual cursor definition
code PGTK-only.
(xw_update_cursor_for_view):
(xw_last_crossing_cursor_cb): New functions.
(xw_maybe_synthesize_crossing): Attach and disconnect cursor
signal handler where appropriate.
(xwidget_init_view): Set the default cursor to the nontext
cursor.
(Fxwidget_delete_view): Disconnect last crossing signal if
appropriate.
* src/xwidget.h (struct xwidget_view): New field
`last_crossing_cursor_signal'.