add afterGetRowHeader event

This adds an event to the rowHeader mechanism, similar to the one used
for column headers. This is needed to implement row moving.
Merge branch 'afterGetRowHeader' into dokuwiki

* afterGetRowHeader:
  add afterGetRowHeader event
adjust row header structure to match column headers

It makes sense to have the same structure for both header types. Makes
developing plugins that shall work on both much easier.
Merge branch 'afterGetRowHeader' into dokuwiki

* afterGetRowHeader:
  adjust row header structure to match column headers