Add placeholders to window canvas

[ggeisler]

Part of #1963

For the window canvas area, the loading state is at least partly represented by the image tiles that progressively load as they become available. But if there is a delay before the tiles become available, we need to convey to the user that something is happening.

Loading state

Until the canvas image tiles begin to load, display a ". . ." loading message centered in the canvas:

Failed state

An image could fail to load on the canvas, such as if the image server becomes unavailable. In this case, at the point we know the image(s) are not available to render on the canvas, we can convey that to the user with an "Item could not be retrieved" message centered in the canvas: