[EuiSelectable] Accessibility issues

[snide]

A follow up issue for the new EuiSelectable component. These were some bits I noticed:

• 1. The tab / down arrow actions aren't clear. In some cases you can use tabbing, in other cases the arrows work.
• 2. Screenreader support could use work. When tabbing, the index isn't read, but when arrows are used they are (sometimes).
• 3. Screenreader needs to read the status of each item. Item is checked, Item is excluded...etc.