Strongly-typed circuit events.

[bitbound]

In Blazor Server, the front-end maintains a persistent SignalR connection to the backend, which is called a Circuit. By default, the circuit is "hidden" from the developer and handled behind-the-scenes by the framework. If you want to send your own SignalR messages, you'd need to create a new client connection that connects to a hub.

Alternatively, you can register your own CircuitHandler derivative, which allows you to add logic to the circuit. I opted for this route to reduce the number of connections. The CircuitConnection class is this custom CircuitHandler, which basically acts as a SignalR client.

The initially implementation was hasty and sloppy. The incoming events had loosely typed data (in the CircuitEvent class), all the front-end state was stored in a singular ClientAppState service, and reactivity was added via untyped property changed handlers.

This PR replaces the CircuitEvent with strongly-typed messages sent via IMessenger, and ClientAppState has been broken out into individual stores.

---

Please read the following. Do not delete below this line.

Thank you for your contribution to the Remotely project. It is required that contributors assign copyright to Immense Networks so we retain full ownership of the project.

This makes it easier for other entities to use the software because they only have to deal with one copyright holder. It also gives me assurance that we'll be able to make decisions in the future without gathering and consulting all contributors.

While this may seem odd, many open source maintainers practice this. Here are a couple well-known examples:

• Free Software Foundation: http://www.gnu.org/licenses/why-assign.html
• Microsoft: https://cla.opensource.microsoft.com/

A nice article on the topic can be found here: https://haacked.com/archive/2006/01/26/WhoOwnstheCopyrightforAnOpenSourceProject.aspx/

By submitting this PR, you agree to the following:

You hereby assign copyright in this PR's code to the Remotely project and its copyright holder, Immense Networks, to be licensed under the same terms as the rest of the code. You agree to relinquish any and all copyright interest in the software, to the detriment of your heirs and successors.

[colinblaise]

Looks good!