This better reflects the pre-release nature of the current state of the code.

We want the server to be able to tell a client to shutdown, e.g. because
there's a server/client version incompatibility after an upgrade

(NB no checks for said compatibility have yet been implemented!)

So far it just copies the build artefacts into an installation dir.

This also lets us remove the slightly clunky warning box
I had permanently on the Status window

Part of #5

These probably got auto-generated by accident when I clicked on the wrong
element in the form designer

This means we can readily set the window icon in a consistent manner to what
we do with the tray icon

Part of #5

If the WCF channel fails/aborts, something bad has happened and
we probably need to have the service restarted. Bail out and
things should work after a reboot.

Resolves #3

Resolves #4

Renders the ugly hack in 4ad59613 unnecessary

This lets us give better feedback to the user about what's
currently happening.

Resolves #1

We need to be sure that some checks are performed in ProcessRequest() and not
call RunWpkgQuery() directly. It feels like encapsulating a private
RunWpkgQuery() inside a CommandHandler class which exposes a public
ProcessRequest() is the right way of doing this... but the requirement to have
shared state between the WpkgInstaller class and that hypothetical class means
that's going to get ugly too. Thus, even though reflectively checking our
caller feels pretty ugly, I'm going with it as the lesser of two evils.

This is a holdover from when the UI was designed around clicking
menu options in the tray icon context menu. That's now all moved
to buttons in the status window.