introspection: add PropertiesChanged signal for o.f.N.Settings.Connection

Some users, e.g. libnm-glib's NMRemoteConnection, use it to get changes of
properties.

introspection/nm-settings-connection.xml

@@ -131,6 +131,14 @@
             </tp:docstring>
         </property>
 
+        <signal name="PropertiesChanged">
+            <arg name="properties" type="a{sv}" tp:type="String_Variant_Map">
+                <tp:docstring>
+                    A dictionary mapping property names to variant boxed values.
+                </tp:docstring>
+            </arg>
+        </signal>
+
     </interface>
 
 </node>