resultsmodel: Read plasma settings for KRunner favorites

This ensures all users of milout have the settings respected.
In case of an explicit assignment, we do not use the automatically read value.
Using the RESET property, one can revert to the automatic behavior.

BUG: 480750


(cherry picked from commit 181013c8)

0e3f7831 resultsmodel: Read plasma settings for KRunner favorites
0e9e38a6 ResultsModel: Override favoriteIds property to only read/assign config value...
5e25d099 ResultsModel: Add RESET to change favoriteIds property back to automanaged