Editor: Add maps module, replace map member of MainWidget with maps::current()
Editor/LayerItemWidget: Replace m_map member with maps::current()
Editor/LayerListWidget: Replace m_map member with maps::current()
Editor/MapWidget: Replace m_map member with maps::current()
Editor/maps: Make current() implementation use a vector
Editor/maps: Add loadFromFile function
Editor: Readd support for creating a new map
Editor: Use maps::currentEntityContainer() instead of reference members