kcms/plasmasearch: Fix sidebar non-default Highlight & Implement restore default favorites

Tobias Ozórrequested to merge
tobiasozor/plasma-desktop:work/defaults-plasmasearch into master

This MR implements restoring the default favorites when pressing the Defaults button and fixes the non-default Highlight which is displayed on the System Settings sidebar.

Bugs fixed

BUG: 523908

Merge request reports