Build and push master branch of the Sdk

Depends upon !88 (merged)

These are cherry-picked commits from the gnome-3-30 branch which allows us to push to the gbm.gnome.org/repo remote.

Tried squashing some stuff, but I don't feel like fighting git for more than 15 minutes. If anyone wants to clean things further feel free to do so.

Part of #49 (closed)