Fix poor choice for FlyoutItem BP Name

[PureWeen]

Description of Change

The idea here was to have an attached property called FlyoutItem.IsVisible that users could use to indicate when they wanted to hide just the FlyoutItem. This works great in code but in XAML when binding to "IsVisible" on a FlyoutItem that will now gets routed to the Attached property that's shadowing the non attached property.

API Changes

Added:

• Shell.FlyoutItemIsVisible //Attached BP
• BaseShellItem.FlyoutItemIsVisible //Attached BP that allows for easy access to this property on each BaseShellItem

Removed:

• FlyoutItem.IsVisible //Attached BP

Platforms Affected

• Core/XAML (all platforms)

Behavioral/Visual Changes

• Removed the attached property FlyoutItem.IsVisible. Users wanting to just hide something in the Flyout will need to use Shell.FlyoutItemIsVisible.

None

Testing Procedure

• unit tests modified
• UI Test Included

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)

[rmarinho]

Related tests were passing