Fix initialization when running from a chain

If the effect was run from a chain without first going into the
effect's dialog, it would error out cause the duration hadn't
been set properly.

src/effects/DtmfGen.cpp

@@ -282,6 +282,13 @@ bool EffectDtmf::Startup()
    return true;
 }
 
+bool EffectDtmf::Init()
+{
+   Recalculate();
+
+   return true;
+}
+
 void EffectDtmf::PopulateOrExchange(ShuttleGui & S)
 {
    // dialog will be passed values from effect

src/effects/DtmfGen.h

@@ -52,6 +52,7 @@ public:
    // Effect implementation
 
    virtual bool Startup();
+   virtual bool Init();
    virtual void PopulateOrExchange(ShuttleGui & S);
    virtual bool TransferDataFromWindow();
    virtual bool TransferDataToWindow();