Optimize wxMSW wxChoice/wxListBox::SetString() if nothing changes

Because the corresponding native controls don't support modifying their
items in place, wxChoice and wxListBox had to remove the item and add it
back, which was quite expensive, so it's better to avoid this if the
item doesn't change at all.

src/msw/choice.cpp

@@ -378,6 +378,11 @@ void wxChoice::SetString(unsigned int n, const wxString& s)
 {
     wxCHECK_RET( IsValid(n), wxT("invalid item index in wxChoice::SetString") );
 
+    // don't do anything if the string doesn't change, deleting and inserting
+    // the item back below is not free and also results in flicker
+    if ( s == GetString(n) )
+        return;
+
     // we have to delete and add back the string as there is no way to change a
     // string in place
 

src/msw/listbox.cpp

@@ -515,6 +515,10 @@ void wxListBox::SetString(unsigned int n, const wxString& s)
     wxCHECK_RET( IsValid(n),
                  wxT("invalid index in wxListBox::SetString") );
 
+    // don't do anything if the string doesn't change, see wxChoice::SetString()
+    if ( s == GetString(n) )
+        return;
+
     // remember the state of the item
     bool wasSelected = IsSelected(n);