Add hasSwapEvent getter

Summary:
Add a small getter to query information internally if the backend supports
swap events. Defaults to true as it is the default in the GBM Wayland backend.

Test Plan: i915

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Maniphest Tasks: T11071

Differential Revision: https://phabricator.kde.org/D25298