SystemDialog was added in 2021 and did not receive any testing while the API was in flux when portals started using it.
Update the implementation to match plasma-workspace now that the API has settled.
Also simplify the UI, remove automatic scrollview and other loaders that make initialization very complicated.
Fixes https://invent.kde.org/plasma/plasma-mobile/-/issues/301
This (partially) reverts commit 127bbdf653.
Only incorrect changes from Kirigami.Units.largeSpacing to
Kirigami.Units.gridUnit are reverted. Only Plasma.Units were buggy.