Change `<a role="button">` controls to `<button type="button">`

[Malvoz]

It's recommended to use <button> instead of <a role="button">. We should change the use of <a> in all places we've introduced them:

• Context menu items
• Popup navigation controls
• Reload button
• Remove Layer button
• ...?

This would require a CSS reset to make buttons look seamless by default, e.g.:

.mapml-button {
  background-color: transparent;
  border: none;
  border-radius: 0;
  color: inherit;
  font: inherit;
  line-height: inherit;
  margin: 0;
  padding: 0;
  overflow: hidden;
  text-align: left;
  text-align: inline-start;
  text-transform: none;
  -webkit-appearance: none;
     -moz-appearance: none;
          appearance: none;
}

(This generic button class was originally proposed in #263 (comment).)

Info on <a> vs <button>:

• https://marcysutton.com/links-vs-buttons-in-modern-web-applications
• https://bitsofco.de/anchors-vs-buttons/
• https://www.w3.org/TR/2016/WD-wai-aria-practices-1.1-20160317/examples/button/button.html

This issue relates to #19 (same problem but keeping the issues separate as that one pertains to Leaflet's controls).