.gitlab-ci: sync the gimp-2-10 CI with master.
It's minimal. I've not added CLang build or the Windows cross-builds yet. Probably later. :-)
Only dependency changes from master are: GTK+2 instead of GTK+3.
Also GIMP is now built with --disable-python because even though Python 2 is still in Debian testing, apparently python-gtk2 is not packaged anymore. So let's unfortunately drop this in our CI for simplicity sake.
This makes both master and gimp-2-10 CI now very similar, hence will make it easier to fix one or the other.