Find a better way to identify resizeable outputs

[any1]

Currently, we check if the name starts with "HEADLESS-". If it does, we assume that it's possible to set a custom mode on the output and set it according to the client's wishes.

The problem with identifying headless outputs in this manner is that an output can be named pretty much anything, although they're named "HEADLESS-something" most of the time. It seems that we will need to extend either wlr-output-management-v1 or wl_output to provide hints about the output type: something that tells us if it's a physical or virtual output.

A better, albeit more difficult, solution would be to create a completely new protocol to create virtual outputs. Then wayvnc can just create its own headless outputs and therefor knows which outputs are headless. It is then also clear that these outputs are managed by wayvnc.