Major api refactor

See https://invent.kde.org/frameworks/plasma-framework/-/issues/15 for details.

- Make plasmoid a pointer to Applet
- AppletInterface and ContainmentInterface become the mandated root item for plasmoids
- The Applet api is exposed now directly, both as plasmiod and Plasmoid.
- full/compactrepresentation is a direct property of PlasmoidItem defined as plasmoid root item

This is a first part of more things to come, as explained in the code TODOs, most notably still missing are:

* new context action api
* revise what is a Q_SLOT and what isn't
* where the screen geometry api goes
* killing packageurlinterceptor
* killing colorContext

BUG: 469143