magnifier: Use inhibit-unfocus API to keep wayland focus while hidden

Since commit mutter@a2a8f0cd we force the focus surface of the meta-wayland-pointer to NULL while the pointer is hidden. This introduced an issue with the magnifier, where we use set_pointer_visible to hide the real cursor and show our own cursor at the correct position: Because the meta-wayland-pointer is still used to communicate with Wayland clients, the UI of the windows will not respond to mouse movement anymore as soon as the real cursor is hidden.

To fix this, use the newly added inhibit-unfocus API of ClutterSeat to temporarily disable unsetting the focus-surface while the magnifier is hiding the system cursor.

Depends on mutter!1077 (merged)