Use resourceClass as fallback for XWayland clients instead of resourceName.

Summary:
resourceClass is the more useful half of WM_CLASS. It's what modern X
clients are supposed to set to a .desktop file name, which is closer to
the semantics of appId than resourceName.

Reviewers: Plasma, graesslin

Subscribers: kwin, plasma-devel, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D5815