KKeySequenceItem: make it possible to record Ctrl+Num+1 as a shortcut.

Summary:
KQuickControls contains a copy of KSequenceWidget but with the UI
handled by a QtQuickControls based QML file.

This syncs the relevant part of the changes from D6226

Test Plan:
Ran test to show widget.
Could press combos

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D6255