Add indication for when waiting on an image to be loaded.

[ArturKovacs]

The user doesn't know if an image is currently being loaded. This is problematic with large images that take several seconds to load because the user gets no feedback when stepping to the next image until the image has finished loading. (More precisely the only feedback is the slider moving at the bottom, but that may be hidden for some and it doesn't feel quite enough)

See also: #197

This was originally created due to https://github.com/ArturKovacs/emulsion/issues/10#issuecomment-617449380

[ArturKovacs]

The example from #197 https://upload.wikimedia.org/wikipedia/commons/1/1f/Geneva_from_Saleve_0.jpg

I think the best solution to this is to display the image while it's being loaded. That is, when half of the image is loaded, then half of the pixels are displayed.