The dialog's presentation mode.
When set to [enum@Adw.DialogPresentationMode.auto], the dialog appears as a bottom sheet when the following condition is met: `max-width: 450px or max-height: 360px`, and as a floating window otherwise.
Set it to [enum@Adw.DialogPresentationMode.floating] or [enum@Adw.DialogPresentationMode.bottom-sheet] to always present it a floating window or a bottom sheet respectively, regardless of available size.
Presentation mode does nothing for dialogs presented as a window.