Game Over

Added a game over condition to the game. When all party members are dead
the game state "GAME_END" is set to "game_over"; this initiates the
"Game Over" convo in SystemDialogue.xml, which sets "GAME_END" to
"quit_game"; setting BaseGame::_keepRunning to false and exiting the
game.

The game is now paused at the beginning of dialogue and unpaused at the
end, and all text is cleared from the dialogue box at the end of
dialogue.