Upgrade layout file to support Doxygen >=1.8.

Upgrade layout file to support Doxygen >=1.8.

Remove entries in the layout file that cause warnings with Doxygen >=1.8
Split the layout file changes into two parts to allow upgrading Doxygen
without breaking existing builds; after this change, the layout file
does not produce warnings with either version, but needs additional
changes to fully conform to what at least 1.8.5 expects (those included
in Ie265644).  Jenkins needs to be updated for this to be useful; until
that is done, some links etc. in the generated documentation may subtly
break.

Also fixed a stray Doxygen warning with 1.8.5.

Related to #1239.

Change-Id: Ieb7d4441ba8f6e6ffde042e20fc51598b3029439