[GUI] Default theme for paksets

[Yona-TYT]

As we already know, the main paksets have their own "GUI themes", so with this experiment I want to try that these dedicated pakset themes are used by default whenever the checkbox is activated.

Any help with this welcome!

There is a problem with the "addons" directory, I don't quite understand the logic to decide whether to use the directory from the simutrans executable or the "pakset" directory in addons.

The pakset directory must have a subdirectory called "theme" and there will be the "themes.tab" file and the respective pak that will be the default theme.