[accessibility] Keyboard operable controls

[Malvoz]

For keyboard accessible controls:

• The controls (print control and those in easyPrintHolder) can be made focusable using either href="#" or tabindex="0" (or otherwise use <button> instead of <a>)
• The controls should activate on Enter and Space press

Resources that you may find helpful:

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