Unselect all wxDataViewCtrl items when clicking outside of the item area.

This is consistent with Windows behaviour and as the generic wxDataViewCtrl is
mostly used under Windows, it makes sense to follow Windows convention in it.

Closes #15082.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73602 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775