Pause on focus loss

[mimblybimbly]

Resolves #191

This is a PR to add an option to auto pause when window focus is lost. The option currently defaults to false. It will pause the game if the window loses focus, and resume if it regains focus (user clicks on window, or alt-tabs to game). Similarly it works in full-screen mode, if the user alt-tabs out of the game it pauses, and resumes if they alt-tab back.

I also experimented with having the loss of focus result in the game showing the pause menu, as if the player had pressed start, but obviously that only works in game-play, and the game wouldn't pause until cut-scenes, transitions or dialog had finished. If people would rather it behave like that, I can add that instead.

[ffdiego]

In my opinion it would be a good feature to add a START keypress on
case SDL_WINDOWEVENT_FOCUS_LOST:
that way the game doesn't immediately resumes after alt-tabbing back, and cutscenes remain paused as well.