kcm: Implement non-default (sidebar) highlights

Tobias Ozórrequested to merge
tobiasozor/krdp:work/default_hightlight into master

SettingStateBinding requires setting names to match the exact casing defined in the kcfg file. Additionally, correct the non-default highlight logic for certificate(Key) text fields.

Implement the System Settings sidebar highlight as well.

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