Add empty state message to load window

[ggeisler]

Part of #1963

It's possible that a user can trigger the load window but there are no pre-defined resources to load. If that's the case, we should show the message "Your resource list is empty" centered in the load window area.

Ideally, when the load window is in this empty state, we would move the "Add Resource" button from the bottom corner to below the empty state message. This would make it a bit more convenient for the user to get started on adding the resource.

If moving the button in the empty state is not feasible, we can leave it where it normally is:

(Note in the mockups the background is not the white of the load window, because we're assuming that the white background comes from the surface of the load window list items and the load window surface itself will be "surface - dark" or #f5f5f5 in the light theme specified in #1906. We might need to update the load window separately for that.)