Right now we just assume that every compositor and portal implementation
exposes both window and monitor captures, but that's not true, and in fact
the Desktop portal provides a simple mechanism to check which source types
are available: a D-Bus property called "AvailableSourceTypes".
Read this D-Bus property, and use it to conditionally register the desktop
and the window captures.
Related: https://github.com/obsproject/obs-studio/issues/4815
