Enhance mission levels select window

[winterheart]

Pull Request Type

• GitHub Workflow changes
• Documentation or Wiki changes
• Build and Dependency changes
• Runtime changes
  • Render changes
  • Audio changes
  • Input changes
  • Network changes
  • Other changes

Description

This PR was inspired by #485. Now missions that contains more that one level, shows enhanced level select window (see screenshot). This enhancement allows user to select available level instead of input number of mission manually. Additional refactoring made for filename paths used in related functions.

Related Issues

Screenshots (if applicable)

Checklist

• I have tested my changes locally and verified that they work as intended.
• I have documented any new or modified functionality.
• I have reviewed the changes to ensure they do not introduce any unnecessary complexity or duplicate code.
• I understand that by submitting this pull request, I am agreeing to license my contributions under the project's license.

Additional Comments

[JeodC]

Nice work, it looks great! Tested in Windows. Do you mind if the core missions go to the top of the list? See here for reference. I also really like the idea of the LEVEL SELECT header being the name of the current mission, as a reference for the user. This was done in the old PR that inspired this current one.