a11y: Can't tab into DropdownTrigger

[rakeshpai]

Since the dropdown trigger is a link without an href, most browsers don't make it focusable, thus it's not possible to tab to the dropdown with a keyboard. This makes the dropdown trigger and all its contents inaccessible to keyboard users.

I found that a previous issue (#29) removed the href attribute, which makes sense in that context.

I could take a stab at a PR, if we agree that this needs to go back in. However, we'll need to decide what a href value should be.

Personally, I don't like using either '#' or 'javascript:void(0)', because the former requires a event.preventDefault() so as not to mess up browser history, and the latter is basically inline JS being eval'd, which makes it incompatible with CSP. In such situations (link that's only used for interaction, not navigation), I usually prefer to use a <button> element instead. Not sure what you'd think about changing the trigger to a button though.

If you don't want to change to a button element, we can stick to a href of '#' with a .preventDefault() on the link.

[takempf]

@rakeshpai Sorry I took forever to get back to you about this. This is a completely reasonable and a pull request would be welcome.

[leonid-shevtsov]

I think a <button> is more appropriate for the dropdown trigger.