Titlebar improvements

[niels9001]

Description

This PR introduces the following changes

• Adds an icon to the titlebar to resolve: [UX] Add app icon to titlebar #1177
• Sets ExtendsContentIntoTitleBar = true; in App.xaml.cs to resolve: [UX] win32 titlebar briefly shows #1178
• Removes a bunch of unused visual states (for e.g. Xbox and Mobile) and XAML.
• Replaces the C# logic for changing the titletext margins with simple visual states in XAML.

[gregwoo-microsoft]

Thank you for implementing this improvement!