Update and extend links to mysql documentation

[DZunke]

I have updated some of the documentation links because they ended at the start page of the MySQL Source Code Documentation.

For the ConstEventType i linked directly to the mysql github repository because the documentation itself seemed not to allow the direct link to the list.

For some links in the project, like the "Event Meanings" in the Readme and the EventDTO class i did not found a corresponding page. I do not know what the original page contained so ... i left them but maybe an update here would also be nice in the future.

[krowinski]

tx