KPluginFactory: Allow registering of classes without QVariantList arg

In a few cases, like applets, the underlying framework needs the
arguments. Most of the time however, they are unused.
With this change, consumers no longer need to take the args in the constructor
and discard them/pass them to the frameworks baseclass where they are discarded.

The usecase I have specifically in mind is KCMs that get shown
in a KPluginWidget. Only in some cases (like KDE-Connect) we
use the arguments. In the KCModule class we don't use them either.