settings: Add function to check if the gsettings schema is installed

gsettings causes the process to abort when the schema is not available with no way to catch the condition (without a signal handler). This means kwin and gnome-shell window managers crash if our gsettings schema happens to not be installed/compiled. This is apparently the intended behavior: https://bugzilla.gnome.org/show_bug.cgi?id=651225

This MR adds a function that allows checking beforehand if the schema exists. This allows window managers to print a warning if it is not, and then simply failing to start the VR mode instead of crashing.

Troubleshooting steps are on the wiki: https://gitlab.freedesktop.org/xrdesktop/xrdesktop/wikis/GSettings-Schema-Troubleshooting