Store cache data of plugins in cache location

[bastiK]

We now have good separation of preferences, user data and cache content for JOSM core, but plugins just put all their files in one location (on Linux that would be ~/.local/share/JOSM/plugins/MyPlugin/).

If any plugin were to produce a lot of caching data, then we would have the same problems as reported for JOSM core earlier:

• Automated backup process saves a lot of junk or needs custom excludes
• Impossible to keep user directory on a network device (for sharing settings, etc.)

I'm suggesting to have 3 dedicated directories for each plugin, on Linux, that would be:

• ~/.config/JOSM/plugins/MyPlugin/ plugin preferences
• ~/.local/share/JOSM/plugins/MyPlugin/ plugin user data
• ~/.cache/JOSM/plugins/MyPlugin/ plugin cache files

[bastiK]

In 13007/josm:

fixed #15436 - Store cache data of plugins in cache location