Addon Manager: Preview image for addons

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Problem description

Currently addons in the addon manager are displayed with a list containing the icon and the name, a click on them reveals the readme.md info for that addon. Most addons require a restart, so there is no quick way to preview a new WB, the functionality of a macro or the look of a theme.
The readme info is very individual, some have text only, some have images, instructions how to install or use it and documentation or help info.
It would be great to have the ability for each 3rd party addon to provide a dedicated preview image, which could be displayed in the addon manager to provide a quick preview of what the addon looks like or what it is intended for.

Regarding themes or workbenches it would be great to have a bigger preview image or gallery, maybe on the new start page or for the first-run wizard so users can get a look before installing and restarting and I think this could be a more unified appearance.

Maybe guidelines and default sizes for addon previews could be provided as a prerequisite.

### Full version info

```shell
0.22
```

### Subproject(s) affected?

Addon Manager

### Anything else?

_No response_

### Code of Conduct

- [X] I agree to follow this project's Code of Conduct