GtkHeaderBar: Display a back button in closable resizable transient dialogs on mobile
When a dialog is resizable, on mobile it will be maximized to completely cover the parent window. Though, on mobile close buttons don't make sense, and closable dialogs should instead mimick a regular view from the application.
To do so, if a dialog is transient, closable, and resizable, it should replace its window decoration by a back button on mobile. Remain the question of when we should consider we are on mobile, see #2190 where this question started being discussed.
Edited by Adrien Plazas