Set event object for wxEVT_DATAVIEW_SELECTION_CHANGED events in wxGTK.

The event object was always NULL for these events before as we didn't set it
at all.

Closes #13478.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75138 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775