Use wayland platform plugin on Gnome

Summary:
QWayland is automatically used on all desktops besided Gnome, but it already works just fine there. We also
should use a gnome platform theme when possible (e.g. QGnomePlatform) for better Gnome integration. Using
wayland makes Qt apps on Gnome wayland sessions to better support HiDPI and also applications using
--socket=fallback-x11 will not fail to start

Reviewers: apol

Reviewed By: apol

Differential Revision: https://phabricator.kde.org/D22829