New level, menu screen, the lot

In the latest build:

• After the last wave, all enemies must be dealt with in order to complete a level.
• We've made some futile attempts at balancing. We'll definitely update rules before the final jam version.
• Added a new, harder level with two enemy spawn points.
• The game starts with a super ugly menu screen with some info on controls and level selection buttons.
• Minor performance optimizations.