Code cleaning + Make sure issues similar to #587 won't happen with AOT.

Description of the changes:

• Make TitleBar generic and remove ApplicationViewModel dependency, so other apps can use this control
• Force the calculation of the title bar visibility when we enter and leave AOT.
• Use Visual States instead of bindings to manage the 2 states of the title bar.
• (very) small performance improvement: deferred loading of ExitAlwaysOnTopButton
• Make sure ExitAlwaysOnTopButton has the same height than the title bar (the height was fixed)
• Remove the converter and properties not used by ExitAlwaysOnTopButton (FontWeight, AllowFocusWhenDisabled, Grid.Row)

In ApplicationViewModel:

• Use macro to declare IsAlwaysOnTop
• Make IsAlwaysOnTop and DisplayNormalAlwaysOnTopOption read-only

How changes were validated:

• Manually