VSDX: import lastModifiedBy

Change-Id: I5136c53cbca0b3dc1300e49bf82f290d8ff86ed8
VSD: Visio doesn't write separate author and last modifier fields

So author always equals to the last modifier. This is consistent with
how the single timestamp is imported both as a creation and modification
time, too.

Change-Id: I65233316f412166dfd82c720c830cf625e1275b5