The VisManager is a singleton holding the visualization layers and providing
the drawing cycle of them.
The VisManager has to be explicitly initialized by the init() method. Without
its calling, the registered layers will be held, but the Vis singleton will
not be used, so the visualization window will not be created and shown. This
feature enables to use a live code for registering of the layers, but with
conditioned turning on of the visualization window only by the
If the manager is not initialized, the drawing cycle is not started, so the
layers do not request any data from the visualized elements.
Besides the registration and un-registration of the layers, the manager also
provides saving of a visualization state into a image file (the saveToFile()