Set embedder correctly to fix menus appearing in the wrong location

Set embedder correctly to fix menus appearing in the wrong location

* src/xwidget.c (record_osr_embedder, from_embedder, to_embedder): New
functions.
(Fmake_xwidget): Attach from-embedder and to-embedder signals.
(find_widget_for_offscreen_window): New function.
(xwidget_button, xwidget_motion_or_crossing): Set embedder on event.
(Fdelete_xwidget_view): Remove embedder status if applicable.

* src/xwidget.h (struct xwidget): New fields `embedder' and
`embedder-view'.