Centre-of-mass indicator and bug fixes

This week not much outwardly apparent has changed with the game but I’ve fixed a load of bugs, including a fairly major error in the gear constraint code that sometimes caused gear trains to gain too much angular velocity.

I’ve added an indicator that shows a construction’s centre-of-mass when you freeze it.  This will be useful when you want to balance a construction around a pivot point or centre-of-lift (e.g. flying vehicle, see-saw, etc.).