[accessibility] Convey the disabled state to screen reader users

[Malvoz]

When the controls are disabled (leaflet-control-navbar-back-disabled, leaflet-control-navbar-fwd-disabled) they should be conveyed as such to screen reader users.

This can be achieved using aria-disabled="true".

Alternatively, the controls could be taken out of the focus order (remove href="#"), and set to aria-hidden="true".

[Malvoz]

Other resources that you may find helpful:

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