Remove old hacks for UTF8 mode.

wxWidgets' UTF8 mode wasn't thread-safe, so we had to put mutex's on
a bunch of string operations, which meant that we couldn't return
references to strings.  Now that we no longer use UTF8 mode anywhere
(and the mutex's were removed long ago), using references is a big
performance win in some places.

pcbnew/footprint.h

@@ -420,7 +420,7 @@ public:
     /**
      * @return reference designator text.
      */
-    const wxString GetReference() const
+    const wxString& GetReference() const
     {
         return m_reference->GetText();
     }
@@ -442,7 +442,7 @@ public:
     /**
      * @return the value text.
      */
-    const wxString GetValue() const
+    const wxString& GetValue() const
     {
         return m_value->GetText();
     }