Fix inserting first choice item to wxEnumProperty

Situation when when first choice item is inserted (in wxPGProperty::InsertChoice) to the empty collection must be handled in a special way. In order to do so:
1. The state when there are no choice items in wxEnumProperty object and hence no item is selected must be explicitly indicated (by special index value wxNOT_FOUND).
2. This initial state must be handled in a special way when there is determined new selection index after insertion.

See #16401.


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76974 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775