Type alias ManualPageID for wxString used in many places...

... eliminating many direct mentions of wxString.  A real type distinction will
be made next.

src/effects/ToneGen.cpp

@@ -139,11 +139,11 @@ TranslatableString EffectToneGen::GetDescription()
       : XO("Generates a constant frequency tone of one of four types");
 }
 
-wxString EffectToneGen::ManualPage()
+ManualPageID EffectToneGen::ManualPage()
 {
    return mChirp
-      ? wxT("Chirp")
-      : wxT("Tone");
+      ? L"Chirp"
+      : L"Tone";
 }
 
 // EffectDefinitionInterface implementation