Anchor group menus to center of app icon buttons

Currently the group menu popup is anchored to the edge of the app icon button, similar to the behaviour of a right-click/context menu (see the screenshots below). However, this looks strange as most other OSes and desktops with taskbar previews anchor this popup menu to the center of the app icon button instead.

This MR automatically anchors the group menu popup to the center of the app icon button both in horizontal and vertical panel layouts, as such:

It also works when window previews are disabled, like so: