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
-

modules/onscreengui/include/gui.h

@@ -69,6 +69,7 @@ public:
     GuiPropertyComponent _globalProperty;
     GuiPropertyComponent _property;
     GuiPropertyComponent _screenSpaceProperty;
+    GuiPropertyComponent _virtualProperty;
     GuiTimeComponent _time;
     GuiIswaComponent _iswa;