ScreenCast: Allow recording new streams on active sessions

This is useful if you have a session, and want to "hot-plug" new sources
over time; there is no point in having to create separate sessions for this.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2131>

data/dbus-interfaces/org.gnome.Mutter.ScreenCast.xml

@@ -190,6 +190,14 @@
   -->
   <interface name="org.gnome.Mutter.ScreenCast.Stream">
 
+    <!--
+	Start:
+	@short_description: Start new stream
+
+	Start a stream of an already started session.
+    -->
+    <method name="Start"/>
+
     <!--
 	PipeWireStreamAdded:
 	@short_description: Pipewire stream added