Use mangle_argument_name for implicit version argument

This is a fixup for commit 1de43d5. The implicit version argument was
hardcoded to 'version_' instead of going through mangle_argument_name.
Fix whitespace errors in generated comment blocks
sample: Generate html docs if Doxygen is available

If Doxygen is found then a make target doc is added which generates html
documentation for the sample code.
Improve generated Doxygen comments

* Add \brief to summaries
* Use @param in event handler signatures
* Add @param for implicit version parameters