Clean up embedding api

Clean up embedding api

we now use gdk_offscreen_window_set_embedder() instead of a signal
to get the parent. This also replaces set_has_offscreen_changes.

Rename "parent" in all embedding related names to "embedder" to make it
more obviously different than the normal parent.

Rename gdk_window_get_offscreen_pixmap to gdk_offscreen_window_get_pixmap
to match the other offscreen calls.

Rename gdk_window_offscreen_children_changed to gdk_window_geometry_changed
as this is more descriptive.