ContextMenuController should creates its FX objects lazily

Today ContextMenuController creates all its FX objects in its constructor.
This makes EditorController construction slower.

ContextMenuController should defer its creation operations until contextMenu is needed.

This could be done in getContextMenu() :

    public ContextMenu getContextMenu() {
        if (contextMenu == null) {
             … create everything…
        }
        return contextMenu;
    }

… then all the remaining code should use getContextMenu() in place of "this.contextMenu".