Porting for Qt 6/KF6

Currently, this provides a base for the application to at least somewhat function.

Draws significantly on initial work by @davidedmundson in !67. Commits to `work/plasma6` welcome!

TODO:
 - [X] CI _- by making KAccounts optional like KUserFeedback_
 - [X] Make KCM pages work (including verifying functionality for:)
   - [X] Feedback
   - [ ] Accounts (not yet available)