KeyWorksRW
Switch to wxFileName for filename processing.
Description:
With the switch to wxWidgets 3.3 with UTF8 set for all strings, the advantage of using ttString for handling filenames and directories using the std::filesystem functions is mostly gone. wxFileName does use more memory and requires more processing but wxUiEditor isn't using filenames or directories in a way where the additional memory/slower performance would have any impact on the user experience. Switching to wxFileName also makes the code easier to read/maintain for anyone not familiar with the ttString class.
wxFileName will use backslashes in paths on Windows -- those will need to be converted to forward slashes if they are saved in a project or XRC file so that they remain platform-independent (Windows can handle forward slashes so it actually isn't necessary to use backslashes).
Note that switching to wxFileNmae to retrieve environment variables like $WXWIN my solve the crash in #1560 -- or at least eliminate it as the cause of the crash.
