Training mode which halves the physics speed and disables time saving

[jukibom]

Should be reasonably easy to add, a toggle which sets the Time.timeScale to 0.5.

[jukibom]

A start has been made to add the basic functionality to the pause / unpause etc with a new game mode but we need to think through how this would be properly enabled. I suspect a bespoke game mode is probably not the way to go as it would be distinct in the level format - maybe a toggle in Options which has the same score legality check as modifying the ship parameters.