Make Gradle aware of XTC

[lagergren]

Create a plugin that treats xtc as a first class citizen language, like Gradle can do for Scala, Java and other custom languages. This is the first step towards IDE integration with debuggers and things like that. Most importantly, it will decouple most of the serial and explicit dependencies in our build, and a lot of the build logic, which exists today, because no system is aware of what "x" and "xtc" files are, can just fold away as dependencies and tasks for all known language

[lagergren]

Pretty much code complete. Can bli simplified a little bit later and incrementally. I want to post my PRs for the examples ported to use this as well, before merge