[libtaskmanager] Track stacking order and window activation (on X11)

Summary:
TaskGroupingProxyModel::requestToggleMaximized now uses this to
minimize and restore groups of windows while preserving the
stacking order, a frequently user-requested wish.
BUG:368867

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.
CCBUG:370258

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.

Reviewers: Plasma

Subscribers: davidedmundson, pino, anthonyfieroni, ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22053