Re-implement autostart functionality

Autostart is now controlled by a KDED module which inspects the version number of the last Plasma
release when the app was run (or notices if it's absent) and does the right thing:
- Autostart the app in welcome mode if it's absent
- Autostart the app in update mode if it's present for a prior version
- Do nothing otherwise

BUG: 459180
FIXED-IN: 6.0