[client] Perform ConnectionThread::roundtrip through QPA interface

Summary:
In case the ConnectionThread is created from the QGuiApplication the
roundtrip is performed through a function provided by the QPA plugin.

If the QPA plugin does not provide such a function, the normal
wl_display_roundtrip is used.

This is mostly a helper for KWin. Plugins might call roundtrip from the
main gui thread which results in a dead freeze of KWin. By providing this
function through the native interface KWin can ensure it doesn't freeze
and the plugin still gets the functionality of the roundtrip.

Reviewers: Plasma

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1152