Make Compositor more verbose

Summary:
In order to help with debugging why the OpenGL scene is not loaded,
it can be really helpful to know what scenes the Compositor attempts to
load.

Test Plan: Ran kwin with QT_LOGGING_RULES="*.debug=true".

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D29261