Improve controls in popups

[Malvoz]

Fixes the following item in #247 (comment):

• Popup's Close button (and controls in mapml-focus-buttons) should be at least 44 by 44 CSS pixels (relates to Increase default size of controls #215).

Also replaces the <div> container for mapml-focus-buttons with <nav>.

Comparison

Before

After

[Malvoz]

Also comitted 9612783 to set role="dialog" on popups (does not entirely fix but is one step of #354) to improve the UX for screen reader users.

[prushforth]

I notice that the content of the popup is not read by NVDA. But it's starting to work better I think. Thanks for this!

[Malvoz]

I notice that the content of the popup is not read by NVDA.

That is caused by #467...