Fix helicopter upgrade strafe when facing SW
Add levels, UI, canvas scaling
Add test level with unlimited enemies
Use mouseup instead of click in UI for responsiveness

Also minor cleanups.
Move bullet out of bounds checking into Bullet object
Optimise collision check counts slightly

More selective checking lists, move bullet-enemy collision check into
bullet.
Clean up Entity initialisation.

I hope!
Added decals!
Add background images, button CSS improvements
Added shadows and light flashes!