Add signals for offscreen window embedding

Add signals for offscreen window embedding

3 signals are for offscreen windows
get-offscreen-parent: Get the parent window an offscreen is embedded in
to-parent: Convert coordinates from offscreen to parent
from-parent: Convert coordinates from parent to offscreen

1 signal is for the window embedding offscreens:
pick-offscreen-child: This picks what (if any) offscreen is at a specific position

The last signal is only used if you call gdk_window_set_has_offscreen_children
to tell gdk that the window has embedded offscreen children.
Add get-pointer signal for offscreen window pointer getting

Apps using offscreen windows can connect to get-pointer on offscreen
windows in order to make gdk_window_get_pointer() return correct
values.

Add get-offscreen-parent signal

Add signals for from-parent and to-parent coordinate mapping

Add pick-offscreen-child signal