Add import statements for PlasmaQuick

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

Although there's one issue, what exactly is ConfigModel - is it public or private API? I'm not familiar with this, but it has this warning at the the top of the file and yet it is visible on Doxygen, exposed via PLASMAQUICK_EXPORT. I added an import statement just in case though, but let me know and I can either peel it off of Doxygen or remove the public API warning, just a little confusing to me.