Refactor ProjectSettings

[madmiraal]

To clean and simplify the code and make it easier to read and maintain in the future, this PR refactors the ProjectSettings class.

• Restructures files to make the public interface clearer
• Initialises variables in the header
• Renames variables, private methods, typedefs, etc. to better reflect their purpose
• Simplifies and merges internal structs
• Removes unused methods and variables
• Makes const methods const.
• Simplifies methods to make them easier to understand
• Makes error messages clearer and comments relevant