vala: unify builders for various versions

We are currently carrying a number of vala versions where each version
is essentially just a copy of the earlier version.

This PR gets rid of a ton of duplication and uses a standard builder.

Secondly, we add a definition for the latest vala 0.34.1.

Lastly, we add a generic "vala" that refers to the latest stable
version.

I have tried changing the definitions for "simple-scan" and "valum" to use
the latest vala version and they at least compile OK so I'll try a
massive sed job to replace all the definitions later to simply use the
latest version through "vala" instead of specifying a version directly.

According to upstream:

"Well-maintained packages are expected to always build with the latest
stable Vala version."

Maybe this means that my generic builder is then no longer necessary. Oh well...

I added myself to the maintainer array for vala although I have no
interest in the language - this was purely a nix exercise for me but I
thought it was reasonable to be the one to clean up the mess if this has
side effects...

Cc: @antono and @lethalman