Merge branch 'master' into v0.6.0

# Conflicts:
#	src/ifcconvert/IfcConvert.cpp
#	src/ifcgeom/IfcGeomShapes.cpp
#	src/serializers/ColladaSerializer.cpp
#	src/serializers/schema_dependent/XmlSerializer.cpp

src/serializers/GeometrySerializer.h

@@ -83,6 +83,14 @@ public:
     const SerializerSettings& settings() const { return settings_; }
     SerializerSettings& settings() { return settings_; }
 
+    /// Returns ID for the object depending on the used setting.
+    virtual std::string object_id(const IfcGeom::Element<real_t>* o)
+    {
+        if (settings_.get(SerializerSettings::USE_ELEMENT_GUIDS)) return o->guid();
+        if (settings_.get(SerializerSettings::USE_ELEMENT_NAMES)) return o->name();
+        return o->unique_id();
+    }
+
 protected:
     SerializerSettings settings_;
 };