shellDBus: Add a real error reporting system to InstallExtensionRemote

Instead of using the 'extension-state-changed' signal to relay errors,
use DBus's native error mechanism to inform the method caller that the
call has failed. This requires making the method actually asynchronous
so that we don't block the browser, which is stuck waiting for a reply
from the browser plugin. To ensure this, we need to modify the browser
plugin API to ensure its extesion installation method is asynchronous.

Additionally, this lets us remove the awful, broken hacks that we used
when a user clicked the "Cancel" button, replacing it by a DBus return
value.

https://bugzilla.gnome.org/show_bug.cgi?id=679099