qt: wait for scene graph initialization in compositors at the latest stage recovery is possible

If scene graph initialization fails, the interface won't be functional. We need a blocking method so that we can fail early in makeMainInterface().

This also fixes the crash observed with direct composition when trying to load video before interface is ready, as a corollary outcome.

This reverts !5950 (merged), and incorporates the changes from !5913 (closed). This supersedes !6010 (merged).

Request review @chub.