In order to use a PanelMenu.Button in the bottom bar, we have to tweak its menu a bit. We currently handle this inside the indicator, but that means the code diverges from the original code in the workspace-indicator extension. Avoid this by using a small subclass that handles the adjustments. Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/307>