Monocle: added IDocument::GetBackendPlugin().
Monocle: added IBackendPlugin::IsThreaded().
Monocle FXB: optimized paragraph handling.

Cut test doc load time by ≈⅓: from 3.6 seconds to 2.4.
Monocle FXB: a huge optimization of typical docs.

Multiple consequent <p> items in a <section> are now handled at once,
leading to around ×5 speed up on my test document.
Monocle: (untested) possibility of threaded rendering.