Improvements and Balance Changes

Due to feedback I'm rolling this update, to improve overall experience.

List of main changes:

- You can now shoot, by holding "SPACE" instead of having to tap it, whenever the shot is ready it will be released

- It's possible to navigate in UI with keyboard. Use movement keys to select upgrades and confirm the choice with SPACE

Balance changes:

- Tweeked numbers on shooting speed, increasing it in early levels

- Upgrades avaible on level finish are now more impactful