X11: set only monitor as primary in a single output configuration

This PR makes it so, in the randr (X11) backend when there's a single monitor, that monitor will always be marked as primary.

It also takes care of two TODOs about checking if the config backend supports the PrimaryDisplay property.

Tested and all pipelines seem to be passing just fine.

This fixes a edge case created by either a race condition or an error when writing the config (Issue #33 (closed)).

I was debating whether we actually needed this, but I think we do as this should be the intended behaviour in X11 regardless of what settings the user has.

Closes #33 (closed)