* changed FBXImporter to return G2::Entity class rather than std::shared_ptr<FBXMesh>
 * extended FBXImporter to get an optional pointer to a G2::Entity to import the mesh instead to the returned one
 * added logic for selecting entities in editor
 * added first implementation of MeshPropertyTab having a separated UI file and class implementation with a foldable design for the tab content
 * added new event for entity selection changed
 * added file selection for mesh tab
 * added drop down with core properties already contained (will be dynamic and extendable later)