Updated developer manual to include full documentation of the event system (not including the event reference).

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@5608 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9