Add optional headless fallback output

Add optional headless fallback output

This allows configuring a headless fallback output that
is automatically created whenever there is no other output
around. It is destroyed when a new output is discovered.

It can be enabled by setting the environment variable
LABWC_FALLBACK_OUTPUT to the desired output name.

The feature benefits applications like wayvnc the most
as there is always an output available to connect to.

Co-Authored-By: Simon Long <simon@raspberrypi.com>