overview: Provide public access for search provider registration

Provide a reasonably public way to register and unregister search
providers, without adding too much API.

`Main.overview.searchController` provides access that may be generally
useful, while `SearchController.addProvider()` and
`SearchController.removeProvider()` provide a simple interface for
extensions.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2783>

js/ui/overview.js

@@ -102,6 +102,10 @@ class OverviewActor extends St.BoxLayout {
         return this._controls.dash;
     }
 
+    get searchController() {
+        return this._controls.searchController;
+    }
+
     get searchEntry() {
         return this._controls.searchEntry;
     }
@@ -708,6 +712,10 @@ var Overview = class extends Signals.EventEmitter {
         return this.dash.showAppsButton;
     }
 
+    get searchController() {
+        return this._overview.searchController;
+    }
+
     get searchEntry() {
         return this._overview.searchEntry;
     }