Add an optional close button to the info bar

This adds gtk_info_bar_{get,set}_show_close_button.
Similar to the one for the search bar. This can replace Cancel
buttons.

https://bugzilla.gnome.org/show_bug.cgi?id=704274

docs/reference/gtk/gtk3-sections.txt

@@ -2381,6 +2381,8 @@ gtk_info_bar_set_message_type
 gtk_info_bar_get_message_type
 gtk_info_bar_get_action_area
 gtk_info_bar_get_content_area
+gtk_info_bar_get_show_close_button
+gtk_info_bar_set_show_close_button
 
 <SUBSECTION Standard>
 GTK_TYPE_INFO_BAR