guuid: Add UUID helper functions to GLib

Many UUID users will just need a random string, which can be generated
simply by calling the function g_uuid_string_random().

Based on original patch by
Marc-André Lureau <marcandre.lureau@gmail.com>

https://bugzilla.gnome.org/show_bug.cgi?id=639078

docs/reference/glib/glib-docs.xml

@@ -94,6 +94,7 @@
     <xi:include href="xml/testing.xml" />
     <xi:include href="xml/gunix.xml" />
     <xi:include href="xml/windows.xml" />
+    <xi:include href="xml/uuid.xml" />
   </chapter>
 
   <chapter id="glib-data-types">

docs/reference/glib/glib-sections.txt

@@ -3403,3 +3403,10 @@ g_hostname_is_ascii_encoded
 <SUBSECTION>
 g_hostname_is_ip_address
 </SECTION>
+
+<SECTION>
+<FILE>uuid</FILE>
+<TITLE>GUuid</TITLE>
+g_uuid_string_is_valid
+g_uuid_string_random
+</SECTION>