Simplify checks for event vetoing in generic wxDataViewCtrl code.

Simplify checks for event vetoing in generic wxDataViewCtrl code.

Don't return the whole event object from SendExpanderEvent() just to check if
it wasn't vetoed, simply return a boolean value indicating if this was the
case from this function itself.

This makes it both more efficient and easier to use.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68867 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775