Update the styles and HTML to reflect the new views manager concept.
For now, nothing about split/merge functionality is implemented or visible.
The new styles for the outline, attachments, and layers will be added later.
The thumbnail view is now accessible with the keyboard.
This is a precursor to moving the call into a
worker thread to let us use `OffscreenCanvas`. The
current position wouldn't work since we make
transformations to the canvas object after the
getContext call, which isn't allowed for
OffscreenCanvas. Also it isn't allowed to clone or
`transferControlToOffscreen` the canvas after the
`getContext` call.
