SceneTreeWidget: Append ellipsis to "Group selected entities" to signal that further user actions are required. Use object_cast instead of dynamic_cast where applicable.
IComponent::DeserializeFrom: improve error print by proving the component's type name (+ fix typo).
scenes/Canvas/scene.txml: add missing attribute names.
EC_DynamicComponent: Fix buggy code in CreateAttribute for checking existence of an attribute (do not use around 'name' variable (IComponent::name) but 'id'). Route RemoveAttribute call to IComponent::RemoveAttribute in order to have single code path for attribute removal and signaling. Remove TODO for already done task while at it.