search fixes

* Readded wrongly removed patches:
  - 0015-tracker-search-engine-don-t-start-it-if-not-availabl.patch
  - 0018-search-engine-add-locate-based-search-engine.patch
* debian/patches/12_unity_launcher_support.patch:
  - Fix not to break builds when --enable-unity is not set
* Updated patches as per upstream reviews:
  - 0016-search-engine-add-a-recent-search-engine-listing-Gtk.patch
  - 0017-recent-add-function-to-update-manager-on-file-rename.patch