Fix .gitignore and add gwt/war to setup extension so gdx-setup.jar no longer fails.

Right now in the nightlies, `gdx-setup.jar` is broken because of missing `war/*` files, which are ignored by `.gitignore` so they are never committed. Not sure what it was supposed to be, but this fixes that issue, plus it will build correctly and `gradle gwt:gwtDev` seems to work correctly as well.