Fix config saving
Applet: run full Applet::save when a config value changes
Plasma had two timers for configuration saving:
- One applet specific, which eventually calls Applet::save
- One at a corona level, which eventually calls KConfig::save
The former was incorrectly removed (now reverted) because it wasn't hooked up to
anything useful, but the real bug is that it should have been.
When a value changes in the KConfigPropertyMap we need to run through
Applet::save to sync that config loader with our main config. This
implicitly emits the configNeedsSaving which will trigger corona to sync
to disk.
BUG: 481113