Filter component: improvements for clearing a query by keyboard
When deleting tokens by pressing the backspace key on the keyboard, once a full query is deleted, the filter becomes blurred. ![backspace-filter](https://gitlab.com/gitlab-org/gitlab-ui/uploads/e0e8882f0b90dcd3ff728d7d463a966c/backspace-filter.mov) Instead, the filter should remain focused and the dropdown should appear so that the user can start inputing a new query.
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