Update the text part of combobox when changing text of selected item in wxGTK.

Add a call to SetValue() to wxComboBox::SetString() in wxGTK if the item being
changed is the currently selected one. The new behaviour is consistent with
wxMSW and also makes more sense.

Document it too to remove any doubts about what is supposed to happen in this
case.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70880 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775