introspect: Add AnimationsEnabled property

While the gsetting is available for all who needs it, the Shell might
override it given various hueristics. Expose the decision made by the
Shell via a new property.

Intended to be used by gsd-xsettings as well as xdg-desktop-portal-gtk.

This also add a version property to the API, so that semi external
services (xdg-desktop-portal-gtk) can detect what API is expected to be
present.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/757

data/dbus-interfaces/org.gnome.Shell.Introspect.xml

@@ -57,5 +57,19 @@
     <method name="GetWindows">
       <arg name="windows" direction="out" type="a{ta{sv}}" />
     </method>
+
+    <!--
+       AnimationsEnabled:
+       @short_description: Whether the shell animations are enabled
+
+       By default determined by the org.gnome.desktop.interface enable-animations
+       gsetting, but may be overridden, e.g. if there is an active screen cast or
+       remote desktop session that asked for animations to be disabled.
+
+       Since: 2
+    -->
+    <property name="AnimationsEnabled" type="b" access="read"/>
+
+    <property name="version" type="u" access="read"/>
   </interface>
 </node>