More std:: style for wxString...

... Remove all uses of the three-valued Cmp comparison member function, which
returns 0 for equality, except in one place.  Use operators == and != instead.

(C++20's spaceship operator hasn't landed here yet!)

src/PluginManager.cpp

@@ -825,6 +825,8 @@ int wxCALLBACK PluginRegistrationDialog::SortCompare(long item1, long item2, lon
 
 int PluginRegistrationDialog::SortCompare(ItemData *item1, ItemData *item2)
 {
+   // This function is a three-valued comparator
+
    wxString *str1;
    wxString *str2;