Custom “clear” button should restore focus to the input

[jouni]

When using a custom “clear” button element, clicking the button causes the input to lose focus. We should move/restore the focus back to the input when the user clicks/tabs the clear button.

We should also consider if we should automatically add tabindex='-1' to custom “clear” and “toggle” buttons, so that keyboard navigation would work the same with the normal and light versions.

Related to #334