Support enabling numlock on startup

Summary:
This change brings improved num lock support to KWin. The modifier state
is read and also mapped to Qt::KeyboardModifiers. Furthermore the input
config is read and the NumLock key is evaluated. If the requested state
does not match the current num lock state the state is swapped.

BUG: 375708
FIXED-IN: 5.15

Test Plan: New unit test added, no manual test due to lack of hardware

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D16428