kcms/tabbox: Highlight non-default shortcuts

This sets the property on the shortcut widgets when they
should get highlighted: the option is selected in System
Settings and the shortcut is not the default.

It requires a change in `KXMLGui/KKeyShortcutWidget` to
actually paint the highlihgt ring.
(commit 47572c26, KF103)

CCBUG: 459318