Feature: Added Notification Type, Type Visibility

[JannesStroehlein]

I added notification types and a property to opt out of showing the notification type. The generic theme now displays the notification type with a accented icon.

New Notification Type Icons

Things I have changed / added

• added interface definition (INotificationMessage)
• added properties and styled properties to control (NotificationMessage)
• added builder and linq builder methods to set the type and type visibility
• added property changed handler to update the pseudoclasses of the control when the type or type visibility are changed
• removed redundant class that was added by the constructor of NotificationMessage, because there was a more elegant way of selecting the textblocks
• added icons licensed under the permissive "Unlicense" license to the theme
• updated sample project to show the types in actions

Every code that I added, which is exposed by the API, has the proper code docs and most of the internal code I changed is also documented.