This is a new take on c8e2c17a that was reverted at da67bde61a due to entities no more recognised by the browsers. Corresponding thread on the mailing list: https://www.mail-archive.com/lyx-devel@lists.lyx.org/msg213179.html
This patch is a huge cleanup overall, by removing the distinction between HTML and XML entities (the latter arrived due to the DocBook support).
In InsetListingParams, I also changed the mechanism that relied on " to use an XML entity to be consistent with the rest of the code, mostly in case someone looks for HTML entities and wonders why they are still there.
The math parser could not handle multicolumn grids. This is a problem because
there is no true ERT in math (everything is parsed).
Now multicolumn cells are parsed correctly. The display is also somewhat OK,
but apart from that any multicolumn related UI is missing. Since the file
format change is now done the UI can be added at any later point. The most
important part of bug 396 is now fixed: tex2lyx does not create invalid .lyx
files anymore for formulas containing \multicolumn.
I updated the tex2lyx test cases that produce correct output. tex2lyx does
still produce invalid output for the test cases which are not updated because
of the previous format change.
