Use electron-store for preferences and consolidate storage.

[wwitzel3]

#1906 introduces electron-store.
#1538 adds preference UI.
#941 introduced server side preference Frontend Proxy URL
#1885 uses a state.json file to store the last known x,y,w,h for the application window.

Let's consolidate these storage locations. I think storing everything using electron-store makes a lot of sense here.

I think due to some points raised in #1538 this work should happen before we release 0.17.

cc: @GuessWhoSamFoo @mklanjsek

[mklanjsek]

I agree that electron-store is a way to go. It stores data as JSON, supports migrations and can be accessed from both main and renderer process. At this point we have enough usage scenarios listed above to test it against so it's a good time to consolidate persistence.

[mklanjsek]

Also relevant for issues like #1850 which also brings to the table how will preferences be exposed to plugin users

[mklanjsek]

The only remaining issue here is preference for Frontend Proxy URL, everything else is covered in #1957