Refactor shared GK location storage

[eamodio]

• Use XDG env vars on all OSs
• Avoids singletons by using the container
• Remove empty implementations on the web

Refs: #3660 (comment)

[eamodio]

To verify:

Validate that using XDG_DATA_HOME changes the location of our shared gk folder.

[sergeibbb]

I'm taking this for the verificaton

[sergeibbb]

fixed in c39ec45

[sergeibbb]

My testing on MacOS:

1. ✅ It keeps using ~/.gk if it exists (despite XDG_DATA_HOME is defined or not).
2. ✅ It creates in ./Library/Application\ Support/gk if ~/.gk does not exist and XDG vars are not defined.
3. ✅ When XDG_DATA_HOME is defined and ~/.gk does not exist it uses XDG_DATA_HOME

I'm marking it as verified.

[github-actions]

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