headerbar: Remove user-settable title

Use window title, or custom title widget if it's set. Remove 'title'
property.

Update demos and tests to set the title on the window instead of
headerbar.

examples/sunny.c

@@ -15,8 +15,6 @@ new_window (GApplication *app,
   gtk_window_set_icon_name (GTK_WINDOW (window), "sunny");
 
   header = gtk_header_bar_new ();
-  gtk_widget_show (header);
-  gtk_header_bar_set_title (GTK_HEADER_BAR (header), "Sunny");
   gtk_header_bar_set_show_title_buttons (GTK_HEADER_BAR (header), TRUE);
   gtk_window_set_titlebar (GTK_WINDOW (window), header);