Editing Slideshow Block to add more images

#### Steps to reproduce the issue

1. Insert a slideshow block into a page or post
2. Reload the page
3. Click on the slideshow block and click the pencil icon to edit it
4. When you select images, it will start off 1, 2, 3, replacing the images that were previously selected.
BUT! If you:
5. Click cancel
6. Click on the slideshow block and click the pencil icon to edit it once more
7. Then you can select images and continue the numbering sequence (ie 38, 39, 40, etc.)

#### What I expected

When editing I didn't think it would "forget" the images you previously selected.

#### What happened instead
It started to replace the images instead of adding to the existing images in the slideshow.

**Screenshots**
If applicable, add screenshots to help explain your problem.
Here's a video I took showing the issue: https://d.pr/i/a9Bmot/83qmvNmO9N

No file or test is named in the issue. Start by reproducing the slideshow block editing flow described in the steps and trace the block's image-selection entry point. Done means selecting images after reopening the editor adds to the existing slideshow instead of replacing its images.

Written by the indexing model from the issue text.