Allow HdyHeaderBar to not show a back button on small dialogs
Currently, closable headerbars on dialogs with a back button (because they are subviews) can display two headerbars: one to go to the previous view, and one replacing the close button, to close the dialog.
While on mobile we only want the the button to go to the previous view, we may want to keep the close button on desktop. To do so, we need some way to ask the headerbar to not replace its close button by a back button to close the window, but to instead just not show the close button.
CC @exalm @bertob
Edited by Alice Mikhaylenko