Update doxygen to 1.8.13

Highlights:

* Add NVARCHAR as a SQL type
* Add mscgen images to index.qhp
* Add sql syntax highlighting to code blocks
* Add support for more CSS formatting and column/row spanning in markdown tables
* Added missing language value for SQL to XML output
* Adds plantuml support Qt compressed help file
* Made the RTF output honor the PAPER_TYPE option.
* New classes for generated HTML div elements.
* Option for PlantUML configuration file
* Removed x flag from util* source files
* Replaced section marker before members by diamond shaped bullet

+ many bug fixes and translation updates

(prlw1)