gdk: Add gdk_seat_get_master_pointers()

gdk: Add gdk_seat_get_master_pointers()

Traditionally (and on most backends) there's a single master pointer driven
by all pointing devices. The notable exception is Wayland though, where
master pointing devices are created per capability in the case of
pointer/touch, and one for each drawing tablet.

This function call makes it easy to access all these.

https://bugzilla.gnome.org/show_bug.cgi?id=790920