Create new versions.yml include file

This is based on !1452 (closed) and includes @nanonyme's original commit.

This changes the branch in the following ways:

• Only expose freedesktop-sdk-* namespaced variables, do the assignments to other variable names locally (this is not really necessary, we could just as well update the references to these branch and snap-branch variables to use the ones defined in the include file, but let's leave that to a later internal cleanup/refactor).

• Add a comment to the include file to better express what it's for, at least

• Include the file in project.conf only under the variables: section, this ensures that:

  • Include files do not have too much power to modify anything in the project.conf, things could easily get out of hand if we allowed this
  • It is more clear when reading project.conf how things come together, rather than having includes at the toplevel and having to follow each include in order to discover what aspects they are modifying.

This MR replaces !1452 (closed)