Allow using wxGridCellEnumEditor with the mouse.

Previously the combobox was immediately dismissed when the mouse was used.

Fix this by copying the code from wxGridCellChoiceEditor to this class. This
is obviously not ideal and both classes should be refactored to avoid this in
the future but for now this at least fixes the user-visible problem.

Closes #13943.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71933 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775