[Popover] Keyboard navigation enhancements

[mirualves]

Feature request summary

Currently, tabbing forward out of popover closes it, sending focus back to the button that triggers it. The expected behaviour is to send focus to the forward element instead.

Partially fixes: https://github.com/Shopify/polaris-ux/issues/161

Rationale

General keyboard Interaction

With focus on the button (trigger):
Enter or Space: opens the popover and places focus on the first menu item.
Tab: Moves focus to the first menu item.
Tab (out of popover): Moves focus to the next element on the page.
Shift Tab: Moves focus to the previous menu item or element on the page.

Try the above instructions on https://www.shopify.com/. It behaves as expected.

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[mirualves]

cc @sadiesaurus