Helper component should be queried from the root level

[tomivirkki]

HasHelper interface's default implementation for getHelperComponent() does a query for [slot='helper']. It may result in unintentionally returning the helper component of a child component.

Example:

<vaadin-custom-field>
  <vaadin-combo-box>
    <div slot="helper">Helper</div>
  </vaadin-combo-box>
</vaadin-custom-field>

CustomFieldElement's getHelperComponent() would in this case return the helper of <vaadin-combo-box>.

Change the implementation to only search for helpers on the root level.

[mshabarov]

@alvarezguille do you have an idea what is the severity and impact of this issue?