TASK-6: setup MainMenuScreen

- add label to TextField
- add static list of available games
- open LevelScreen when game is clicked, handing in selected player
- display message to selected user on LevelScreen
- MISC: clear buffer on render()
- MISC: update Viewport on resize()
- MISC: add convenience method in BaseScreen to get Skin
Merge pull request #11 from nathanmyles/TASK-6

TASK-6: setup MainMenuScreen