Allow broader use of property tables in reference topics

[robander]

Stage 1 proposal from Jang: https://lists.oasis-open.org/archives/dita/201803/msg00072.html

Discussed April 3 2018, left at stage 1 until a voting member is able to take this on as a stage two proposal.

[robander]

Discussed again at TC meetings April 23 and April 30.

Right now, <properties> can be used as a child of the reference body <refbody> and the division<refbodydiv>. There are a couple of options to make it more widely available:

1. The reference syntax section <refsyn> can already use simple tables, so it can easily accommodate property tables. Adding to that model is basically a bug fix.
2. Re-architect the specialization so that property tables are now a domain, pull them from the content model of the current 2 elements, and reconfigure the shell so that it imports them as a domain. This would require everyone who has a configured reference shell (and wants to continue using property tables) to update their own shell, so definitely not backwards compatible.

Kris queried the dita-users list about how widespread use of the element is, and got so little response that there were suggestions maybe the TC should remove the property table element (and its 8 descendants).

Given that response, the current consensus is to add the tables into <refsyn> and treat that update as a bug fix. It is a trivial update with no backwards compatibility issues.

If someone on the TC chooses to push the issue, there may be a separate full proposal to remove this element from the OASIS defined reference topic. If that happens, the set of elements could still be added back in as a domain by anyone who needs the tables; doing so would effectively give them option number 2 above, and fulfill the suggestion originally made in the stage 1 proposal.

[robander]

Implemented with oasis-tcs/dita-techcomm#30