This improves on the original fix to #5285 by preventing
other mixed lists (lists with a mix of Plain and Para
elements) that were allowed given the original fix.
Closes#5285. Previously the algorithm allowed list items
with a mix of Para and Plain, which is never wanted.
compactify in T.P.Shared has been modified so that, if
a list's items contain (at the top level) Para elements
(aside from perhaps at the very end), ALL Plains are
converted to Paras.