gitlab-ci: new build organization.

Rather than having the whole Win32 cross-build into the 'gimp' stage,
break the dependencies and GIMP-only builds in 2 stages.

Since apparently we need to keep the same structure for the native and
cross build (otherwise we don't get parallel builds; in other words, I
didn't find the possibility to set separate pipelines up), I move babl
and GEGL into the same 'dependencies' stage.

Finally I remove the -base rules extended into actual jobs, except for
`.gimp-base` (this is the only which makes sense as it is actually
common to the meson and autotools build).
5 jobs for master in 23 minutes and 12 seconds (queued for 1 second)
Status Job ID Name Coverage
  Dependencies
passed #454548
deps-git

00:03:02

passed #454549
deps-x86_64-w64-mingw32

00:11:32

 
  Gimp
passed #454550
gimp-autotools

00:11:39

passed #454551
gimp-meson

00:04:48

passed #454552
gimp-x86_64-w64-mingw32-meson

00:10:47