Add QFilePath

Everywhere in Qt, we use QString for file names. This has several disadvantages:

1. you cannot overload functions for string and file(name)
2. if we want to support std::filesystem::path, we need to overload all function that take QString-as-file-name
   • for a particularly horrible peek at how this might look, see Qt 6's QFileInfo API
3. simple file-name manipulations like basename or dirname require use of the rather expensive QFileInfo
4. Everything always has to go through UTF-16, even on platforms, such as Unix, which do not ascribe meaning to the octets that make up file names, with very few exceptions.
5. Case-sensitivity has to be thought of manually

Enter QFilePath, which could solve all of the above:

1. overload QString and QFilePath, naturally
2. (eventually) have one QFilePath overload that accepts both QString-as-a-file-name, as well as {{std::filesystem::path}}s
3. simple textual conversions can be as cheap as QString modifications
4. underlying storage could be the platform's native encoding instead of UTF-16 everywhere
5. operator== could have default case-sensitivity built-in

This could be an inline wrapper around std::filesystem::path, if we can depend on its presence on all platforms.