More property work

Add StringProperty
Add possibility for callbacks
Switch RenderablePlanet to use Properties

include/openspace/properties/templateproperty.h

@@ -44,6 +44,9 @@ public:
     operator T();
     TemplateProperty<T>& operator=(T val);
 
+    void setValue(T val);
+    T value() const;
+
 protected:
     T _value;
 };