One Property Tree (#500)

* Organize properties in one single property tree
* Update scenes to work with one property tree. Fix documentation issues.

include/openspace/engine/moduleengine.h

@@ -28,6 +28,7 @@
 #include <memory>
 #include <vector>
 
+#include <openspace/properties/propertyowner.h>
 #include <openspace/util/openspacemodule.h>
 
 #include <ghoul/misc/assert.h>
@@ -47,8 +48,10 @@ namespace scripting { struct LuaLibrary; }
  * Additional OpenSpaceModule%s can be registered with the #registerModule function, which
  * will internally call the OpenSpaceModule::initialize method.
  */
-class ModuleEngine {
+class ModuleEngine : public properties::PropertyOwner {
 public:
+    ModuleEngine();
+
     /**
      * Registers all of the OpenSpaceModule%s which are created by the CMake configuration
      * and stored in the <code>moduleregistration.h</code> file. For all of those modules