Limit interactions in maps without layers

[Malvoz]

As I was reading "Maps have one or more layers" in @prushforth's <layer-> doc draft I realized I didn't know what would happen in maps that don't have any layers (or layers without src and no inline content).

After inspecting how browsers (Chrome and Firefox) treat <video></video> I believe we should try to limit interactions with the map element as much as possible if it has no layers. For example by:

1. Disabling controls (aria-hidden="true" and tabindex="-1" on each control)
2. Disabling the context menu (and keyboard shortcuts)
3. Hide the predefined attribution

[Malvoz]

2. Disabling the context menu (and keyboard shortcuts)

The context menu should most likely not be disabled, especially if that's the only way for users restore removed layers (#460).

[prushforth]

So far, I don't see how we can get back a deleted node or restore the checked attribute states of the DOM. Unless the browser is considering a new DOM history API that allows you to capture DOM states not just the page URL.