Recalculate initial folder at startup if it's deleted

[Florian]

@Florian

Instead, I'll try to do the follwing: if the directory to open on startup isn't available anymore, Mp3tag will fall back to its temporary folder. This is also the folder used when the option to open with an empty folder is set at Options → Directories → Startup directory.

Then, on File → Change directory or Ctrl+D while the current directory is the temporary one, the open folder dialog will be initialized using the Favorite directory set at Options → Directories. If that's empty or also doesn't exist anymore, Mp3tag will fall back to the Music directory.

I've just released Mp3tag v3.30a, where I've implemented the new approach described above.