Need state segments for connection statuses

[ethanjli]

Right now connections/disconnections are only reported in the event log. In order to have selectors in the frontend, etc., which can respond to disconnection events, we should report connection statuses as protobuf messages. e.g.:

message BackendConnections {
    bool has_firmware;
    bool has_frontend;
}

We already have a reducer for the store to track when the frontend detects that it has the backend vs. doesn't have the backend: it's in store.app.backendConnection.