schema: Add "locate-pointer"

Initially, "locate pointer" was handled in gnome-settings-daemon using a helper application which would work only on X11, not on Wayland.

While moving the feature to mutter/gnome-shell, however, the previous setting in "org.gnome.settings-daemon.peripherals.mouse" is to be moved to "org.gnome.desktop" instead.

However, having that setting in "org.gnome.desktop.peripherals.mouse" is a bit too restrictive, because this is about the pointer and not just the mouse (the device).

So add the new key "locate-pointer" in "org.gnome.desktop.interface" as suggested by Carlos Garnacho.

mutter!453 (merged) gnome-shell#981 (closed) gnome-settings-daemon!86 (merged)