Fix cursors on some GNOME installations (#12914)

This PR adds support for `org.gnome.desktop.interface`'s `cursor-theme`
setting on Wayland. This should fix cursors not showing up on some GNOME
installs. This PR also adds the wiring to watch the current cursor theme
value.

Thanks to @apricotbucket28 for helping debug the issue.

Release Notes:

- N/A