ComboBox: No visual indication for lazy loading

[d3xter666]

Bug Description

When there's lazy loading, there's no clear indication that the items are being fetched from the backend, for example. Instead, the ComboBox freezes and stays unusable for a while

Expected Behavior

There should be a clear indication that items are being loaded and this delay is not due to ComboBox's malfunctionality.

Steps to Reproduce

1. Go to https://sap.github.io/ui5-webcomponents/master/playground/components/ComboBox/
2. Scroll to section Lazy loading
3. Click on the arrow to load the items