[accessibility] Keyboard and screen reader support for the close button

[Malvoz]

• The control is not focusable, it should be a <button type="button"> or role="button" (with tabindex="0" or <a href="#">).
• aria-label or title can be used to allow screen reader users to understand what the control is for.
  For example title="Close sidebar".

[Malvoz]

Other resources that you may find helpful:

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