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 (closed). Commits to work/plasma6 welcome!

TODO:

• CI - by making KAccounts optional like KUserFeedback
• Make KCM pages work (including verifying functionality for:)
  • Feedback
  • Accounts (not yet available)