initial commit of new LwjglHeadlessApplication for a headless libgdx

A headless version of LwjglApplication called LwjglHeadlessApplication that can be used for servers or other desktop platforms without graphics, audio or input.

I have not tested this extensively.  I only made a very simple libgdx program and confirmed it runs on my VPS.

also some very minor LwjglApplication cleanup:
- replaced HashMap with ObjectMap
- explicitly parameterized Runnable Array objects