If all paragraph margins are 100% on import, ignore that as being the implicit
default. That avoids explicit 100% being set onto the awesome [UL|LR]Space
which takes a relative propsize of 100% as a flag that its value field is
absolute and so rejected by SwTxtFmtColl::Modify as a candidate for getting its
true value initialized relative to its parent, so it ends up as an absolute 0

Always elide the property on export because writing individual
margin-foos provides better backward compatibility with older versions
anyway.

Trigged by 3c5facfc

Change-Id: I55f6ceeae287b7d8e99befa4bd3cc06738a21299