It closes #837 and fixes #822

Also, in case if User opens a wallet and the network does not exist on the discovery service — the app will ask the User to switch the network.
The same behavior will be triggered in case if network disappeared from the discovery service (not sure do we need to do that or not), this check happens every 5 minutes.

About refactoring.

In common, it decouples initialization of Managers (Node, Smesher, Wallet) and some event handlers from the requirement of prefetch data, that can't be prefetched since the user didn't select a network yet. In other words, now Smapp may start without knowledge about any available network, and therefore may start Node and GRPC clients much later.

To reach this goal I had to:

1. Introduces context in the main process. So now different parts of the system may be less coupled with others but have a shared "state" (in the simplest version which mutates an object by reference).
2. Make things work fine without a very special order of function calls
3. Pull out some event handlers from WalletManager, so WalletManager becomes in common the same mediator between client and GRPC Services, as others.
4. Refactor fetching networks, node config, public APIs, and stuff related to this

I believe that these changes will make the app more stable (unknown network / disconnected now is an expected state of the App), and more maintainable since we decoupled a lot of parts of the whole system.

IPC subscriptions, invokers, and types should be refactored further. But probably within another PR.