LyXHTML: switch the doctype to (X)HTML5 and only output XML entities.

LyXHTML: switch the doctype to (X)HTML5 and only output XML entities.

This is a new take on c8e2c17a that was reverted at da67bde61af 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.