Create libgdx directory when fetching

The libgdx fetcher assumes that ProjectRoot/libgdx exists. It does
weird things if it doesn't.

So, we're expressly creating it before cding to it.
Updated desktop build.xml

The classpath to gdx-tools was not correct, causing the build to
ultimately succeed, but to runtime error when the menu is presented.

This updates to the current libgdx-nightly gdx-tools location.