Before this commit, there was a list of perspective grids
kept in CanvasResourceProvider in ViewManager.
It *looked* like a list of perspective grids per canvas,
but in fact it kept all perspective grids from all documents
opened in the same window.
Using that list caused issues like perspective grid in one document
affecting painting in another document.

This commit removes this list altogether.
(Instead, use AssistantsDecoration to get the list of assistants;
currently all KisAbstractPerspectiveGrid implementations
are assistants).