rebased / 1.5

I tried to reach you via mail, but I guess it went under. Since I can't create issues in this repo I created this useless MR:

gamemode 1.5 just got released and I thought about giving the salsa repo a little cleaning, meaning rebasing all the packaging on top of the upstream gamemode git repo instead of manual source uploads. You can find my repo here. With this layout it's also easy to add a debian/buster-backports branch, like they mentioned in DEP-14. Sadly older Debian git tags are lost, it would be possible to recreate each release but I don't think it's worth it.

I also simplified the build process by excluding the subproject. gamemode now builds fully with debuild :)

The only thing where I'm unsure are the pidfs_open calls, which meson tells me are not supported when I build it, maybe there's a new dependency.