mauiproject: Add message box
mauiproject: Fix clear password

Add missing method to Greeter.
mauiproject: Show greeter only for the primary screen

Do a separate instance of Theme for the primary screen so we can
reference it and call methods to set message.

We'll need Theme instances without the greeter for other screens.
mauiproject: Fill the whole screen with a noise background

Otherwise we get a white background with more than one screen when
secondary is bigger than primary.
mauiproject: Use stack view for slide animations

Save time using a stack view that already implements slide animations.