systemdialog: Update to match plasma-workspace implementation, and simplify UI

SystemDialog was added in 2021 and did not receive any testing while the API was in flux when portals started using it.

This MR updates the implementation to match plasma-workspace now that it has settled.

This also vastly simplifies it to improve maintainability, removing automatic scrollview (unnecessary since it doesn't exist on the desktop implementation) and other loaders that make initialization very complicated.

Fixes #301 (closed)