Add possibility to easily set LibGDX version

Developing on nightlies is good for bleeding edge projects. For some projects
we might want to use more stable environment. Added two variables that allows
us to set LibGDX version during project creation. By default, we still use
latest nightlies.

This is probably last pull request in a while, because this closes my list of possible improvements I was missing for private development. I actually cannot think about anything to add right now :)