Site Picker: support esc key to collapse the picker pane

[alisterscott]

Raised by @rickybanister

Currently when switching sites, in order to 'cancel' selection you must find the previously selected site and re-select it. It would be nice if there was a quicker way to 'cancel' the switch.

I think it would be nice to support the use of esc for this purpose.

If you've focused the site search and entered text an X appears to clear the search. You can also at that point hit the esc key to clear it. At that point, I think a second tap of esc would be a convenient way to cancel site switching and collapse the picker—returning to the previous section.

• Just an extra note: we are planning to auto-focus the search by default and if you've entered text in the search I think it would be excessive to collapse the entire picker upon hitting esc once. You should be able to clear the search (with esc) and try a different search. Only when the search field is empty would esc cancel site selection.

[alisterscott]

The above PR #5583 will address this for most browsers and scenarios.

Raised #5771 bug as clearing search field doesn't work on Firefox and MS browsers