Made core flixel more customizable

- new compiler conditional FLX_NO_PAUSE_SCREEN that disables the default
focus lost screen
- new compiler conditional FLX_NO_SOUND_TRAY that disables the default
sound tray
- all keys used by flixel are now fully customizable via FlxG:
keyDebugger, keyVolumeUp, keyVolumeDown and keyMute
- you can turn off the pausing on focus lost by setting FlxG.autoPause
to false now