[plugins/qpa] Add a roundtrip platform function

Summary:
Mostly intended for plugins loaded into KWin and wanting to use
KWayland::Client::ConnectionThread::roundtrip(). By providing this
function we can ensure that KWin does not block and nevertheless
perform the required roundtrip.

Reviewers: Plasma

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1153