feat: Create window for preset previews

[tfuxu]

Feature description

This idea was initially discussed some time ago, but no one has cared before to create an issue for it.
We should create a custom window which will be similar in look to Adwaita Demo application, so that we could showcase as many themeable widgets as possible.

Why should we do it?

Because without our own demo application that could be manipulated by Gradience, we won't be able to override its stylesheet, until the user applies a preset globally (which is how it works currently, if you ever used Preview Preset... menu before, you probably saw that demo apps that we use aren't themed until you click an Apply button in Gradience and reload them).

Additional Context

No response

Did you read the Code of Conduct?

• I have read the Code of Conduct.