keyboard: Add "Keyboard Settings" menu entry

The keyboard popover doesn't have a shortcut to open the Keyboard panel from the Settings app.

This commit adds this menu entry to improve usability.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2600>

js/ui/status/keyboard.js

@@ -857,6 +857,8 @@ class InputSourceIndicator extends PanelMenu.Button {
 
         this.menu.addMenuItem(new PopupMenu.PopupSeparatorMenuItem());
         this._showLayoutItem = this.menu.addAction(_("Show Keyboard Layout"), this._showLayout.bind(this));
+        this.menu.addSettingsAction(_('Keyboard Settings'),
+            'gnome-keyboard-panel.desktop');
 
         Main.sessionMode.connect('updated', this._sessionUpdated.bind(this));
         this._sessionUpdated();