0.4.0_beta: out of alpha & more customization

This is the last main feature I wanted to add to TailTrail.

When getting select achievements in levels, you can unlock cursor skins and animated backdrops! Keep going to obtain them all! Possibly more will be added in future updates.

However, this project is just in the "polishing and bug fixing" phase, and no more major features are going to be implemented.

Hope it's enjoyable!