Convert all wxTRANSLATE() to XO()

XO() can be used anywhere a string must be extracted for translation
but not automatically translated at runtime.

src/effects/Silence.cpp

@@ -36,7 +36,7 @@ wxString EffectSilence::GetSymbol()
 
 wxString EffectSilence::GetDescription()
 {
-   return wxTRANSLATE("Creates audio of zero amplitude");
+   return XO("Creates audio of zero amplitude");
 }
 
 // EffectIdentInterface implementation