This is based on wlroots as of de56ea6b1e3cfa41981fd4dd349b0eef852aee23

Changes made:

 - Drop non rootston parts
 - Move includes and rootston source to src/
 - Rename config.[ch] to settings.[ch] so we can use a config.h
 - Add gitlab-ci
 - Allow to use wlroots as a meson subproject via a git submodule
 - Switch license to GPLv3+. We need to find a way to share changes
   with wlroot since making this one way is not fair.

No attempt was yet made to deny the rootston offspring so the common
prefix is still roots_ but use phoc_ in new files.

Parts of the meson build logic ./meson.build, protocols/meson.build was
taken from sway as of commit d016848bcee829a9db09ed640d075bcff914f06b.