Feature/virtualproperty (#286)

* Add virtual property owner to OpenSpaceEngine

* Ability to add and remove virtual properties

* Do not try to render empty PropertyOwner name
Compile fix for iswaComponent
Add example in default.scene

* Delete virtual properties also in the OpenSpaceEngine dtor

* Make RenderablePlanet not crash if the nighttextures are not present

* - Add VirtualPropertyManager to manage ownership of virtual properties
- Only execute regex when it was necessary
-

include/openspace/engine/openspaceengine.h

@@ -56,6 +56,7 @@ class ParallelConnection;
 class RenderEngine;
 class SettingsEngine;
 class SceneManager;
+class VirtualPropertyManager;
 
 class SyncEngine;
 class TimeManager;
@@ -124,6 +125,7 @@ public:
     properties::PropertyOwner& globalPropertyOwner();
     scripting::ScriptEngine& scriptEngine();
     scripting::ScriptScheduler& scriptScheduler();
+    VirtualPropertyManager& virtualPropertyManager();
 
     
     // This method is only to be called from Modules
@@ -169,7 +171,6 @@ public:
 private:
     OpenSpaceEngine(std::string programName,
         std::unique_ptr<WindowWrapper> windowWrapper);
-    ~OpenSpaceEngine() = default;
 
     void loadScene(const std::string& scenePath);
     void gatherCommandlineArguments();
@@ -195,6 +196,7 @@ private:
     std::unique_ptr<interaction::InteractionHandler> _interactionHandler;
     std::unique_ptr<scripting::ScriptEngine> _scriptEngine;
     std::unique_ptr<scripting::ScriptScheduler> _scriptScheduler;
+    std::unique_ptr<VirtualPropertyManager> _virtualPropertyManager;
 
     // Others
     std::unique_ptr<properties::PropertyOwner> _globalPropertyNamespace;