Fix assert when destroying wxDataViewCtrl being edited in wxGTK.

Fix assert when destroying wxDataViewCtrl being edited in wxGTK.

If wxDataViewCtrl was destroyed while showing a generic editor, an assert
occurred in wxWindowBase dtor as the event handler pushed on it by the editor.

Fix this by calling CancelEditing() when the control is destroyed and also fix
the crash in CancelEditing() in wxGTK due to recursive calls to FinishEditing().

Closes #12683.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67612 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775