[Enhancement] Add 'Clear' button to the dropdown

[jarekmorawski]

We want to make it easy for people to clear all added categories. To do that, we'll add a X button to the dropdown's search field.

Acceptance criteria

• There's a X button visible in the search field when there's at least 1 item added.
• The icon for the button is close-small from the Gutenberg library (color gutenberg-700).
• The X button is visible when the search field is opened (both resting and focused).
• Clicking the X button removes all added items (chips).
• When there's any text input (when the user is searching or creating a new item), clicking the X button will clear the previously added items but not the currently edited text.