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.

GNOME/gnome-shell#981
GNOME/gnome-shell!413
GNOME/mutter!453
GNOME/gsettings-desktop-schemas!19
GNOME/gnome-settings-daemon!86
4 jobs for !413 with locate-pointer in 9 minutes and 15 seconds (queued for 3 seconds)
detached
Status Job ID Name Coverage
  Review
passed #224275
check_commit_log

00:04:04

 
  Build
passed #224276
build

00:02:28

 
  Test
passed #224277
test

00:02:41

passed #224278
test-pot

00:02:39