Change 'IsPasswordRevealButtonEnabled' To 'PasswordRevealButtonMode'

[marwalsch]

The current behaviour of IsPasswordRevealButtonEnabled is:

when false: The inline reveal-button is never shown;
when true: The inline reveal-button is shown only when the input is done on its initial focus while empty.

I do believe it might be favourable to add a third option that permanently displays the inline reveal-button.

enum PasswordRevealButtonMode
{
     Hidden,  // current 'false'
     Auto,     // current 'true'
     Visible   // show permantly
}

This would eliminate the necessity of manually setting up an external control to regulate the reveal mode, as suggested in the WinUI 2 Gallery.