let ConfigModule set KAboutData from KPluginMetaData

This builds on the KPluginMetaData based constructor, which unfortunately isn't terribly useful as modules are generally constructed through the default constructor and the modules themselves have no access to the metadata, so there's not really any way that constructor would get called short of manually loading a second metadata instance.

Instead, the code loading KCMs may now default construct a Module and then still fill in KAboutData from KPluginMetaData through the relevant setter, bringing KPluginMetaData support en par with KAboutData support.

Notably this will allow kcmutils to always set KAboutData when possible, reducing the risk of crashing on nullptr KAboutData and also reducing the amount of information duplication as most KCMs are fairly trivial and will have KAboutData that merely copies what is already in the desktop file.