Improve login UI and general layout

[ivancea]

Login form could be centered in the screen, like a pop-up.

On login, hide it and show the game.

Technically, a UI layer should be added. It could be, for example, an automata-like object, where a controller handles the UI, and on some events, it replaces itself with a different controller. Like:

Form -(On join)-> Game
Game -(On connection error)-> Results
Game -(On end)-> Results
Results-(On play again)-> Game
Results-(On return)-> Form