Display a warning when the installed poppler lib is not at the required level to support the current PDF plugin

[Lewis Rosenthal]

Instead of merely not rendering PDFs, apply check at startup for requisite level of poppler dll. If not found, present a warning panel, e.g.:

Unable to load PDF Plugin.
Possible reason: Incompatible poppler library.

Please install poppler-0.42.0 to enable PDF rendering.

This would work for popplers which are too new as well as those which are too old.