EuiColorPicker focus & popover behavior

[thompsongl]

As discussed in #1914 (comment), we may want to alter the open-on-focus behavior of the popover in EuiColorPicker.

The gist is that a user navigating via keyboard is forced to close the popover when attempting to tab through a form. We'll likely want more explicit open & close actions, similar to EuiSelect

[thompsongl]

Prior discussion:

I don't think this particular use-case needs the dropdown arrow. Someone using the keyboard for navigation should get a VO read out telling them to hit "space" to "enter this input" or whatever. The arrow would be misleading in this case because we use that to indicate there are predefined choices but the color picker isn't necessarily that.

I do think we'd need a visual cue to compliment the VO cue for keyboard navigators not using VO. There's a discoverability problem otherwise; it just looks like a text box.

If you're turning off the auto open, then you'll need something that opens it up. I'm torn between thinking that the caret should be a button or whether it should be purely visual and you hit something like "downarrow" for trigger the popover.