Initial migration to Gtk4

Changes:

• UI:
  • change margin_left, margin_right with margin_start, margin_end
  • remove packing and use widget hexpand and halign properties
  • Remove image and label child from buttons and use the icon_name and label properties
  • Have just one GtkHeaderBar in main window
• Code:
  • Replace dialog.run with callback
  • Use the new calendar widget API
  • Replace box.add with box.append
  • Change all show_all with just show