call: Make ::appendText invokable

For the bindings

Change-Id: I90572880acd3b96ec6442f2ad08024909f2c10a5
Reviewed-by: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>

src/call.h

@@ -347,7 +347,7 @@ public:
    //Mutators
    template<typename T>
    T* addOutgoingMedia(bool useExisting = true);
-   void appendText(const QString& str);
+   Q_INVOKABLE void appendText(const QString& str);
    void backspaceItemText();
    void reset();
    bool joinToParent();