Use .static.isMeta to communicate meta-ness

This gets rid of the meta-specific hack in ve.dm.Document

Later on, I want to use this to get rid of meta-specific hacks in the
converter too, by pushing some of that logic into the ModelFactory.

Change-Id: I1dbee1654fa32d9c7cd521ec325b2553cde219d1
Merge "Use .static.isMeta to communicate meta-ness"