Gravitational Racing 1.4

This update fixes a few issues with the previous version and those introduced by the new 0.19 update as well as some new scenarios and supernovae.

- Fixed track overview not showing in hub-world
- Fixed player being spawned in the wrong place in Layered Instability
- Fixed trails disappearing after resetting a scenario
- Fixed end-screen UI's buttons not working when clicked
- Some code cleanup/general improvements:
- Better handling of laps
- Cleanup of function descriptions
- Adjust portal texture in the hubworld
- Added in a new celestial type - supernova, as well as 2 new scenarios with this celestial type:
- Explosive Force (Advanced)
- Ticking Timebomb (Expert)

Note: these levels are towards the end of the hub-world so will require decent progress to allow you to access them (this is something I wish to address in a future update as I don't feel happy with how it pans out currently)

Supernova example: