Improve label selection performance in Vue Labels dropdown

What does this MR do?

Fixes following issues in labels select dropdown;

• Improve label select performance by not waiting for state update to show ✔ on UI (and do it via local data prop).
  • This change reduced duration of selecting a label (i.e. Update Layer Tree activity) from ~400ms down to ~2ms.
• Prevent dropdown from closing while clearing search input field.
• Fix a bug where select and unselecting same label multiple times causes that label to show up twice in values section.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team