Allow compositors to send discrete axis values

Summary:
So far not all v5 features were implemented because most of them are
optional. But given that XWayland needs axis_discrete event maybe it's
time to implement them.

CCBUG: 404152

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, mthw, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D18933