Bring out GWT version to the DependencyBank. Adds support for the Version page on the LibGDX website.

This PR is required for https://github.com/libgdx/libgdx-site/pull/13 

* Takes GWT version out to the DependencyBank
* Adds VersionBuilder, which is responsible for creating a .txt file with versions of dependencies (used for the Version page)
* Modifies the Ant build to move the generated .txt file to dist