Update to version 0.5.2

Expansion of Classic mode section

Since the story mode update is out, I started working on classic mode again. List of changes below:

- small bug fixes and polishing were done (in both modes)

- small balancing updates on level 8 and level 9 in Greenland forest location
- levels 5 - 8 in Fields of Ashes location were updated, it should be harder to complete them now

New Challenger levels:

- new classic mode menu section was added, you can access the "Ultimate/ Challenger levels" now
- there is currently one new Challenger level available, alongside with a new achievement for finishing it

Controls changes:

- "Attack" was finally moved back to left mouse button, the issue was resolved
- I am currently planning to add options to change the user controls input, it will be probably part of one of the future bigger updates