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 core 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