Broaden GWT constructor support + few minor GWT related fixes.

Resolves all current compiler issues as tested with libgdx-artemis-quickstart. see #137

- create constructors only for public classes
- access protected constructors by using an anonymous class
- fixes various small GWT issues in the current build.

Left some todo's in `World.java` for when we get to use the LibGDX GWT annotation improvements.