Add visibility toggle to Property base class

Restructure GuiPropertyComponent to take a source function that returns a list of PropertyOwner%s instead of registering individual properties
Apply restructuring

include/openspace/rendering/screenspacerenderable.h

@@ -83,9 +83,6 @@ protected:
     glm::vec2 toSpherical(const glm::vec2& euclidean);
 
 
-    void registerProperties();
-    void unregisterProperties();
-
     void createShaders();
     glm::mat4 scaleMatrix();
     glm::mat4 rotationMatrix();