remote-search: add an org.gnome.ShellSearchProvider2 dbus interface

We are going to change the interface, so add a new version of it.
Providers will need to opt-in to the new API.
A summary of the differences compared to the previous API:
- ActivateResult() now also takes the search terms and a timestamp as
  parameters
- a new LaunchSearch() method is added

https://bugzilla.gnome.org/show_bug.cgi?id=689735

docs/reference/shell/shell-docs.sgml.in

@@ -44,6 +44,7 @@
   <chapter>
     <title>Integration helpers and utilities</title>
     <xi:include href="doc-gen-org.gnome.Shell.SearchProvider.xml"/>
+    <xi:include href="doc-gen-org.gnome.Shell.SearchProvider2.xml"/>
     <xi:include href="xml/shell-global.xml"/>
     <xi:include href="xml/shell-wm.xml"/>
     <xi:include href="xml/shell-xfixes-cursor.xml"/>