applets/kicker: Invoke addLauncher/hasLauncher on the PlasmoidItem
In Plasma 6, we have a well-defined root item PlasmoidItem which contains those functions, no need to go hunting for a random object in the tree anymore, which is incorrect now.
- Pinning and unpinning apps from Task Manager works now
- “Pinning” apps to the desktop (create a shortcut in Folder View) works now
- Pin state is correctly reflected, albeit it’s a bit wonky in Kickoff, it seems it caches the menu when opened once and never updates, but that is unrelated to this fix