Authenticator dbus api: Add Webflow signal

When resolving the transactions we call RequestRefTokens as needed
to get bearer tokens for some refs. These calls can also emit
the Webflow signal on the request object with a url. It is then
up to the client to show this url in some way.

Once the required operations are done in the browser it will redirect
to some url that will reach the authenticator, telling it that the
operation is done and the final result of it. At that point the
authenticator will emit the WebflowDone signal and continue.

If the cliend doesn't want to do the web flow it can call the close
operation on the request object.

data/org.freedesktop.Flatpak.xml

@@ -183,6 +183,11 @@
   <interface name="org.freedesktop.Flatpak.AuthenticatorRequest">
     <method name="Close">
     </method>
+    <signal name="Webflow">
+      <arg type="s" name="uri"/>
+    </signal>
+    <signal name="WebflowDone">
+    </signal>
     <signal name="Response">
       <arg type="u" name="response"/>
       <arg type="a{sv}" name="results"/>