Port away from deprecated KWindowSystem API

Summary:
Call the all-in-one method (which handles numeric keypad better),
then split out modifiers and keycode.

Test Plan: Builds, didn't test yet.

Reviewers: zzag, graesslin, davidedmundson

Reviewed By: zzag

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23228