[accessibility] Keyboard- and screen reader-friendly buttons

[Malvoz]

Looking at the demo page, I noticed that:

• the <a> (polyline-measure-unicode-icon) and <div> (close, download-button) controls are missing role="button"
• controls are not keyboard focusable (href="#" / tabindex="0")

Alternatively use <button type="button">.

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Other resources that you may find helpful:

• Leaflet guide to accessibility: Test your maps
• Leaflet Plugin Authoring Guide: Content Accessibility