ODF and OOXML export filters can now check if XML is well-formed

[Michael Stahl]

since there was a bug in this area recently (and OOo issuezilla/fdo
bugzilla indicate it wasn't the first one), i've added checks for
matching start/end tags and unique attributes on master, so we abort
when embarassingly non-well-formed XML is generated.

due to performance impact these are only active in --enable-dbgutil
configuration.

96ee9884b78dce54223e83943a7c5c832ae78759 for ODF
830cc95abb7911f5f6f0ba8c71ef46f3ca8ef383 for OOXML

however there are some short-cuts by which the XLSX export filter
apparently adds attributes that would not be checked for uniqueness.

if somebody has a lot of free time they can try to load and save lots of
docs in such formats and see if some assert hits.