Decide to use the wayland backend by looking at XDG_SESSION_TYPE

Summary:
With plasma-workspace master, QT_QPA_PLATFORM is no longer set,
so spectacle runs with the xcb platform plugin.

Test Plan: Installed, can take screenshots on wayland again.

Reviewers: #spectacle, graesslin

Differential Revision: https://phabricator.kde.org/D11655