Allow stopping the installation

[joseivanlopez]

There is no way to stop the installation once it starts. Sometimes it does not make sense to continue with the installation, for example if there is an important storage commit error. Now, that kind of errors are reported to the user (see #558), but the overall installation process always continues.

Some open questions:

• How to cancel the installation (from D-Bus point of view)?
• How should the UI offer the option for canceling the installation?
• How should the UI behave after canceling the installation?

[mvidner]

From the viewpoint of the lower layers:

The D-Bus specification/protocol does not have an API for ending a service, it assumes a connection simply drops off the bus by its process ending or by closing its socket. Some services implement a timeout, other implement an explicit Quit method.
If we do this we may create a problem where the UI client keeps talking to us, activating another instance with a reset state and not realizing it. If the client wants to detect that, it should use org.freedesktop.DBus.GetNameOwner, save the service unique connection ID (:1.42) and compare it later.

ruby-dbus so far does not care how to disconnect (and perhaps reconnect), it assumes that a connection lives until your process ends (mvidner/ruby-dbus#43). I think it is fine for Agama, just mentioning it for completeness.