output: introduce wlr_output_set_name
wlroots picks names for all outputs, but it might be desirable for compositor to override it.
For instance, Sway will use a headless output as a fallback in case no outputs are connected. Sway wants to clearly label the fallback output as such and label "real" headless outputs starting from HEADLESS-1.
Sway patch: https://github.com/swaywm/sway/pull/6705
Edited by Simon Ser