Add `site_state_dir()` counterpart for `user_state_dir()`

[PhrozenByte]

The user_state_dir() method currently lacks a site_state_dir() counterpart.

Some suggestions:

• Unix: /var/local/$appname:/var/lib/$appname multipath?
• Android: user_state_dir() (as with any other site_*_dir() counterpart)?
• Windows: site_data_dir() with an opinionated State subdirectory?
• Mac OS: site_data_dir()?

This is a prerequisite for #213

[wfjt]

And logs don't belong in cache, they belong in XDG_STATE_HOME as per specification. Annoying to patch it every time I use this lib.