Feature: use specific submodule branches or commits

A new subsection in the build.yml file allows to use specific submodule branches or commits. In the icon section, add a section SUBMODULES. It contains a list of entries, one per submodule, that should point to a non-default value. The syntax is <submodule>:<branch>:<commit>, where submodule is the submodules name, e.g. art, the branch is the branch to check out, e.g. master and commit is either a specific commit or tag or can be HEAD, or CURRENT. HEAD means to use the branches HEAD and CURRENT to use the current state of the repository (if it is not yet existing, use the branches HEAD).