[trunk] Margins of boxes not compatible with previous version (w/ patch)

Issue: In 0.9.6.x, box margins were taken into account only when nesting another box inside the box. When reading 0.9.6.x .msc? files with trunk, texts inside boxes (and possibly other elements) are wrongly formatted because margins are now used.

Patch: The attached patch modifies box.cpp by setting box margins to 0 when reading older files, unless the box contains a nested box. Tested with a variety of 0.9.6.x files.