Initial screenshot support
I got some spare cycles while listening to some online ourse and so i added initial screenshot support. It's as simplistic as it could be as it just always takes a screenshot of the primary output but that's my main use case to e.g. document issues i hit on the road and with the bits in place now we can extend from there.
Print-screen key (phoc!249 (merged)).
Since this also needs
wl_shm buffer handling i added a
PhoshWlBuffer which we can later on use for the toplevel thumbnails as well, i didn't move the code over yet since i'm i think we should at leasat wrap it
Signed-off-by: Guido Günther email@example.com