Headerbar title/subtitle cleanups

• Remove title, subtitle, has-subtitle
• Show window's title by default
  • Remove the logic for updating it from GtkWindow, let GtkHeaderBar do it automatically
• Rename custom-title -> title-widget
• Update many demos to set window title instead of headerbar title
  • Fix peg solitaire in process, it has "sliding puzzle" window title, but "peg solitaire" headerbar title
• Reimplement subtitle manually with 2 labels in GtkAppChooserDialog and GtkFileChooserDialog

See #2242 (closed)

CC @otte