Balance and bug fixes

We've made a few bug fixes, balance changes, and are making music that should be added soon.

- Added some warmup for particle effects to hopefully reduce any lag/freezing on first load of the game.

- Added possibility for a line of trucks to spawn after wave 3.

- Made base attack speed and range higher.

- Added upgrade for attack speed.

- Added a score to ui.

- Make items disappear after they move past 3 times.

- Fix gameover sound to play when paused.

- Made items on the ground stop moving at checkpoints.

- Upgrade cost adjustments.