Can't install to themes under per-user directories

[geefr]

Testing on Linux (mint, 20.04), Stepmania 5_1-new (today).

InstallMenu.js only looks under the stepmania install folder for themes, so misses the case where users installed these to appdata/their home directory.

Can't speak for everyone but I tend to install themes under /home/username/.stepmania-5.1/Themes, so they're preserved across multiple builds/installs of SM 5.1

I think the folders would be:

• Windows: under appdata - As already identified for reading score data
• Linux: /home/username/.stepmania-Major.Minor (I think for club fantastic/etc it'll be a 5.1 base, rather than a special name here)
• Mac: Should be the same as Linux