[accessibility] Delegate focus when context menu has been closed

[Malvoz]

When the context menu closes the focused element is body, this can be a confusing experience for people with screen readers and also requires keyboard-only users to sequentially navigate back through all items on the page before getting back to their original destination (before opening the context menu).

Focus should be delegated to the previously focused item (and if not available, the map container itself should receive focus).

Related issue: #135.

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Other potentially helpful resources:

• Leaflet Guideline: Test your maps
• Leaflet Plugin Authoring Guide: Content Accessibility