Document wxItemContainer::SetStringSelection() as case-insensitive.

Add unit tests checking that the behaviour really corresponds to the
documentation too.

And also mention that it's not a good idea to have strings differing by case
only in wxComboBox anyhow.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66442 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/combobox.h

@@ -23,7 +23,10 @@
 
     Please refer to wxTextEntry documentation for the description of methods
     operating with the text entry part of the combobox and to wxItemContainer
-    for the methods operating with the list of strings.
+    for the methods operating with the list of strings. Notice that at least
+    under MSW wxComboBox doesn't behave correctly if it contains strings
+    differing in case only so portable programs should avoid adding such
+    strings to this control.
 
     @beginStyleTable
     @style{wxCB_SIMPLE}