Tabaccess

Hello.

I notice that the menus are keyboard accessible when the `trigger` is set to `click` by tabbing to the button and hitting `enter` which acts like a `click`. However, when `trigger` is set to `hover` the menu is inaccessible via tab focus. I've had a quick go at fixing this.

What I have done seems to work, except for when you tab to the menu really quickly upon page load (not sure why). Anyway, as imperfect as it is, I thought I'd push this so you can take or leave the idea as you see fit.

Thanks for your work.

Joel.