add waves

[mwcz]

Add waves, aka stages, aka levels. Each level should have fixed difficulty and configurable waves.

Question: should random seed be fixed, so wave plays out the same every time? We can still add a TON of waves so that there's lots of replay value.

Add a new game plus feature where once all waves are completed, celebrate the player's victory, then start them over at wave 1, but with an extra challenge.

[mwcz]

@Jared-Sprague I made an attempt to start implementing waves/stages/levels, whatever we want to call them. I changed existing code somewhat so maybe tomorrow I can show you what I did. Definitely want your feedback here to be sure I took this in the right direction.