Armada - more options

Motherships can now spawn multiple times.
Player can decide how many motherships they want (None, just one, or
multiple at an interval of their choice).
Mothership kills can be a win condition.
Players can choose how many win conditions must be met in order to win.

Aliens can increase in health over time. Code for it is in AlienEntity.
Very thoroughly commented, but I know still it's not easy to follow.

For repair powerup, one of the choices is now to spawn after a
mothership is taken down.
(The plan is to have a unique spawn condition for each powerup.)