Bug fixes, new enemy AI, and more

- fixed inconsistent velocities
- separated enemies to enable custom behaviour (and boss fights!)
- added bombers that drop mines around the level
- the planes now fly around "randomly" and shoot in different directions
- unofficially centered explosions on the entity that created them (may only work on small entities)
- the longer you go without being hit, the more points you get per kill
- added a pause menu
- added controls help on the top right of the screen
- added game to launcher, can now be downloaded from the website
- made scores per-player
- texture change for enemy_3.png