search: Allow searching for people in overview mode

This adds contacts search to shell, powered by libfolks.
Changes:

- Add Folks and Gee to the build system
- ShellContactSystem, a backend in C
- ContactDisplay, search frontend in JS

https://bugzilla.gnome.org/show_bug.cgi?id=643018

js/ui/overview.js

@@ -11,6 +11,7 @@ const Shell = imports.gi.Shell;
 const Gdk = imports.gi.Gdk;
 
 const AppDisplay = imports.ui.appDisplay;
+const ContactDisplay = imports.ui.contactDisplay;
 const Dash = imports.ui.dash;
 const DND = imports.ui.dnd;
 const DocDisplay = imports.ui.docDisplay;
@@ -211,6 +212,7 @@ Overview.prototype = {
         this._viewSelector.addSearchProvider(new AppDisplay.SettingsSearchProvider());
         this._viewSelector.addSearchProvider(new PlaceDisplay.PlaceSearchProvider());
         this._viewSelector.addSearchProvider(new DocDisplay.DocSearchProvider());
+        this._viewSelector.addSearchProvider(new ContactDisplay.ContactSearchProvider());
 
         // TODO - recalculate everything when desktop size changes
         this._dash = new Dash.Dash();