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