locatePointer: Add implementation in gnome-shell

locatePointer: Add implementation in gnome-shell

The "locate pointer" functionality was implemented in gnome settings
daemon using X11 protocols and would fail when run under Wayland.

With Wayland, there is no global coordinate space exposed to the clients
so this functionality cannot be implemented as a separate program.

Instead, add the "locate pointer" functionality in gnome-shell so that
it works in both X11 and Wayland.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/981
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/413
https://gitlab.gnome.org/GNOME/mutter/merge_requests/453
https://gitlab.gnome.org/GNOME/gsettings-desktop-schemas/merge_requests/19
https://gitlab.gnome.org/GNOME/gnome-settings-daemon/merge_requests/86