Add an ultimate fallback to WM_CLASS Class for AbstractTasksModel::AppId in XWindowsTasksModel.

Summary:
This makes grouping work for apps we can't find on the system and
can't produce a launcher URL for, such as wine clients. Any better
app will use our superior codepath, the wonky ones like wine then
get left to making sure their WM_CLASS Class makes sense as per the
ICCCM spec.

Of course this doesn't work for XWayland clients on Wayland though.

BUG:368078

Also nixes an obsolete FIXME and adds a small check to avoid
producing launcher URLs that are only the window icon pixmap
query element.

Reviewers: Plasma, davidedmundson, broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2804