PWMAngband's player reports issue with (same as V) SDL2 client

[PowerWyrm]

How to reproduce the issue:

- open SDL2 client with main window + subwindow
- close client
- subwindow configuration is saved
- open SDL2 client again
- hide subwindow
- close client
- subwindow configuration is NOT saved
- open SDL2 client again with only main window
- show subwindow
- subwindow configuration is back to default

I never touched SDL2 client code, but it's obvious that it's because configuration is saved only if subwindow is visible in dump_window(). This sounds horrible as you have to reconfigure the subwindow each time you hide it...

[backwardsEric]

A fix will likely mean changing the format (new directive or changed format for existing directive) of sdl2init.txt so the configurations for the hidden terms are included. That's only a problem if someone is going back and forth between versions with and without the change: the old version will bail out with a parser error if it reads a sdl2init.txt written by the new version.