GtkAboutDialog: Make credits section extensible

This commit adds API that allows to add new named sections
to the Credits part of GtkAboutDialog, in addition to the
hardcoded sections for authors, documenters, artists and
translators.

https://bugzilla.gnome.org/show_bug.cgi?id=484693

docs/reference/gtk/gtk3-sections.txt

@@ -37,6 +37,7 @@ gtk_about_dialog_get_logo
 gtk_about_dialog_set_logo
 gtk_about_dialog_get_logo_icon_name
 gtk_about_dialog_set_logo_icon_name
+gtk_about_dialog_add_credit_section
 gtk_show_about_dialog
 <SUBSECTION Standard>
 GTK_ABOUT_DIALOG