`TaskGroupingProxyModel::requestToggleMaximized` now uses this to
minimize and restore groups of windows while preserving the
stacking order, a frequently user-requested wish.
Window activation is additionally tracked to implement a new front-
end feature to activate the most recently active window (or fall
through to stacking order otherwise) subsequently.
A Wayland implementation requires the addition of a
`PlasmaWindowManagement::stackingOrder()`, which should be a QList
of PlasmaWindow* in stacking order, along with a change signal. We
discussed this at the Plasma+KWin sprint and I'll code up patches
to KWin and KWayland soon and then implement the new API in here.