[accessibility] Use `<button>` or `<a role="button">`

[Malvoz]

<a class="gps-button"> button is announced as a "link" using a screen reader (missing role="button", see Leaflet controls).

Another option is to use <button type="button">, but requires additional styling to reset browser defaults.

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Other potentially helpful resources:

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

[stefanocudini]

@Malvoz great can you send us a little PR about this? I think is oneline around button constructor code.