With the CMake changes to IrrlichtMt, it's now possible to use a target for IrrlichtMt.
Besides greatly improving the ease of setting up IrrlichtMt for users building the client, it removes the need for Minetest's CMake to include transitive dependencies such as image libraries, cleaning it up a tiny bit. The PR works by finding the IrrlichtMt package and linking to the target it provides. If the package isn't found and it isn't building the client, it will still fall back to using just the headers of old Irrlicht or IrrlichtMt.
* Automated build changes
* drop a useless ubuntu non lts version
* update gitlab ci syntax to modern one
* add debian 10 step on the gitlab ci pipeline
* switch docker build to Debian 10
* add .gradle folder to gitignore
* Implement win32 & win64 builds in Gitlab CI + pkg improvements
* windows buildbot: split build & deploy steps
* move deb artifacts to root folder to have a nicer artifact
* add windows build + packaging on xenial
