keybindings: Do not grab the locate-pointer if unnecessary

On X11, mutter needs to keep a grab on the locate-pointer key to be able to trigger the functionality time the corresponding key combo is pressed.

However, doing so may have side effects on other X11 clients that would want to have a grab on the same key.

Make sure we only actually grab the key combo for "locate-pointer" only when the feature is actually enabled, so that having the locate pointer feature turned off (the default) would not cause side effects on other X11 clients that might want to use the same key for their own use.

Fixes: #647 (closed)