Support multiple running games in client

[Zomis]

It should be possible to switch between games in the client, like on Lichess.org.

This is really helpful for games where opponent is slow, or when you want to have long-running games that can be played over the course of days or even weeks.

To accomplish this, perhaps it is time to use Vuex? This could make it easier to add new features to the toolbar.

[Zomis]

Using Vuex made this a lot easier.