gnome-terminal: enable GNOME Shell search provider (for 3.20)

[This is a re-re-application of the patch that was originally written
for GNOME 3.16. It was lost during the transition to GNOME 3.18 and
then to 3.20. Third time's a charm?]

Silly ./configure, it looks for dbus file from gnome-shell in the
installation tree of the package it is configuring. Fix by copying
needed file from gnome-shell before ./configure is run.

This change makes gnome-shell a build time dependency (not runtime).