[EuiColorStops] Open popover on thumb mouse down

[nreese]

The current EuiColorStops popover behavior is a little confusing. When a popover is open for a thumb and the user drags a different thumb, the popover for the original thumb stays open during the dragging process. This is confusing to show the popover for the thumb that is not being interacted with.

I would expect onMouseDown to close any other thumb popovers and open the popover for the thumb the user is interacting with.

cc @thompsongl