Reorganize pillarbox-demo structure

[MGaetan89]

Pull request

Description

This PR Reorganize the file structure of the pillarbox-demo module. It also renames some file, to make sure that every showcase has the Showcase suffix.

The new package structure more closely matches the UI of the demo app, to make it easier to find sample code:

Changes made

• Reorganize the file structure of pillarbox-demo
• Remove support for Material 3's dynamic color

Checklist

• Your branch has been rebased onto the main branch.
• APIs have been properly documented (if relevant).
• The documentation has been updated (if relevant).
• New unit tests have been written (if relevant).
• The demo has been updated (if relevant).
• All pull request status checks pass.

[StaehliJ]

Maybe for easier understanding the viewmodels can be inside the showcase file. So one showcase will have exactly one file.

#400