Centralise padding values

[MGaetan89]

Pull request

Description

As suggested in #328 (comment), I've grouped every padding values in a common place available under MaterialTheme.paddings.*.

Changes made

• Added an extension on MaterialTheme to provide a Paddings instance with every supported paddings
• Replaced every padding values in pillarbox-demo with the values from MaterialTheme.paddings

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.