GdkWindow: Fix gdk_window_set_[device_]cursor() issues with root/foreign windows

GdkWindow: Fix gdk_window_set_[device_]cursor() issues with root/foreign windows

It could be the case that gdk_window_set_cursor() is called on
pointers not yet known to the device tracking code in GdkDisplay,
so update the cursor on all master pointers.

The code actually updating the cursor for the given window has
been refactored out to gdk_window_set_cursor_internal(), used
in gdk_window_set_device_cursor() as well, which makes it handle
root/foreign windows too.

https://bugzilla.gnome.org/show_bug.cgi?id=649313