Pause screen and scene transitions

Scenes are no longer additive when changing scenes but instead replace
each other.
Added a pause screen to local game that can be used to return to the
main lobby.  Escape key to open.
Online pause

Added pause menu in online games.
Pausing disables player input and provides clickable options.
Player can go back to main menu from pause screen, exiting the game, but
proper server disconnecting isn't in yet.
removed unused variable