Convert Handler Events to Changing/Changed

[PureWeen]

Description of Change

Attaching/Attached/Detaching/Detached are difficult to interpret when a Handler is changed to a new handler. Using a "Changing" approach that has the Old and New handler on it provides a more useful context for action.

For Changed we are still just using EventArgs because at the point that you've hit changed it's not recommended to access the previous handler. So if users want to access the old handler it's on them to do so and understand the risks

Additions made

• Adds Microsoft.Maui.Controls.HandlerChangingEventArgs
• Adds HandlerChanging
• Adds HandlerChanged
• Removes Attaching/Attached/Detaching/Detached