Make Scratch Pad more discoverable

[MartinZikmund]

Description

Scratch Pad is a very useful feature allowing for quick XAML testing, but it is currently buried under System and not easily discoverable.

This PR brings it into the FooterMenuItems which makes it more prominent.

Alternatively it could instead be added under the "Accessibility" menu item, not sure which option is preferrable:

Motivation and Context

• The app provides many XAML code snippets, so users are likely to want to try some variations of those

How Has This Been Tested?

On local machine

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)