quickSettings: Introduce QuickSettingsMenu

The quick settings menu is a popover that arranges items in a
reflowing, homogeneous grid. Grid children may span multiple
columns, but not rows.

For now the QuickSettingsMenu that contains the grid is just a
convenience wrapper around the layout manager that does the heavy
lifting. The two will become more intertwined when we add support
for menu toggles though, so the custom menu type is unfortunately
needed.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2392>

js/js-resources.gresource.xml

@@ -93,6 +93,7 @@
     <file>ui/pointerA11yTimeout.js</file>
     <file>ui/pointerWatcher.js</file>
     <file>ui/popupMenu.js</file>
+    <file>ui/quickSettings.js</file>
     <file>ui/remoteSearch.js</file>
     <file>ui/ripples.js</file>
     <file>ui/runDialog.js</file>