Slideshow and Carousel Blocks - Incorrect Number of Items Displayed

### Quick summary

When editing the number of items in a Slideshow Block or Coblocks Carousel Block, the number of items in the media library or the slideshow/carousel will be displayed incorrectly before updating the gallery

This bug is a display error only. It doesn't adversely affect the use of the blocks.

https://user-images.githubusercontent.com/12906034/209423443-7a45836e-3e61-43e9-b761-13392ae959a5.mp4

### Steps to reproduce

1. Insert a Slideshow or Carousel Block.
2. Edit the Slideshow or Carousel to add some images.
3. Select an image to add, then select Add to Gallery.

### What you expected to happen

It will display "showing X of Y media items" where X is the number of images that are selected, and Y is the number of images that used to be in the gallery.

### What actually happened

It should display "showing X of X media items"

### Context

Discovered during unrelated testing of Carousel and Slideshow Blocks.

### Platform (Simple, Atomic, or both?)

Simple, Atomic

### Theme-specific issue?

_No response_

### Browser, operating system and other notes

_No response_

### Reproducibility

Consistent

### Severity

All

### Available workarounds?

No but the platform is still usable

### Workaround details

_No response_

Reproduce the issue in the Slideshow and Coblocks Carousel block editors, then trace where the media-library count is displayed after selecting Add to Gallery. Done means both blocks show the selected-item count as X of X before the gallery is updated, without changing the platform's existing usability.

Written by the indexing model from the issue text.