No way of temporarily displaying a grid and removing it, as clean up isn't implemented

Hi,

Thanks for a great library! :-) I've created some proof-of-concepts at work using the older version.

Right now I'm looking to open a "widget" (terrible name I know) that contains a grid. The user can close the widget. Therefore, it needs to have its resources cleaned up.

`grid.stopPaintThread()` and `grid.stopResizeThread()` are not hooked up correctly and throw.

I can call `grid.canvas.stopPaintLoop()` successfully to stop hooking into `requestAnimationFrame` instead.

However, there is currently no way to remove the `document` level events that the canvas hooks up. This causes exceptions to be thrown once the element containing the div is removed from the page.

See: https://github.com/fin-hypergrid/core/blob/master/src/lib/Canvas.js#L53

Start with src/lib/Canvas.js at line 53 and trace the document-level events attached by the canvas, along with grid.stopPaintThread(), grid.stopResizeThread(), and canvas.stopPaintLoop(). The work is done when a grid can be closed, its resources and document listeners are cleaned up, and removing its containing element no longer causes exceptions.

Written by the indexing model from the issue text.