configure-printer: Clean up and port for Qt6/Plasma6

Remove the features/functions that the KCM now has for the basic printer configure for an existing printer; the first "page" (Printer Settings) in the configure-printer app, which also includes the make/model selection dialog (which, is also duplicated in libkcups).

Also clean up some of the headers with forward declarations.

Do not merge until !63 (merged) is merged

re: #5 (closed)