Add import statements to org.kde.plasma.core documentation

Adds cleaner looking import statements, along with the versions exposed to the custom QQuickItem classes in PlasmaCore. Useless @class defs are removed as well.

I split this into parts because I'm getting tired of long MRs, and now couldn't find my patch that overhauled all of the modules, so I'm kinda over it now :-) I might end up doing multiple modules in one MR since some of them are small though.