kcms/rules: Notify when a window does not provide a class

Some applications do not provide a window class according to
the spec (WM_CLASS on X11, appId on Wayland), so KWin cannot
"detect" this property.

In those cases, notify the user that this is a bug within the
application (so it is not confused for a KWin misbehavior)
and also prevent setting a wrong description.

BUG: 462644
FIXED-IN: 5.27