Add link support to GtkLabel

This patch is based on SexyUrlLabel, but with significantly enhanced
functionality: keynav, tooltips, context menu, theming.

docs/reference/gtk/tmpl/gtklabel.sgml

@@ -162,6 +162,24 @@ aligns in its available space, see gtk_misc_set_alignment().
 
 </refsect2>
 
+<refsect2>
+<title>Links</title>
+
+<para>
+Since 2.18, GTK+ supports markup for clickable hyperlinks in addition
+to regular Pango markup. The markup for links is borrowed from HTML, using the
+<tag>a</tag> with href and title attributes. GTK+ renders links similar to the
+way they appear in web browsers, with colored, underlined text. The title
+attribute is displayed as a tooltip on the link. An example looks like this:
+<informalexample><programlisting>
+gtk_label_set_markup (label, "Go to the &lt;a href=\"http://www.gtk.org\" title=\"&amp;lt;i&amp;gt;Our&amp;/i&amp;gt; website\"&gt;GTK+ website</a> for more...");
+</programlisting></informalexample>
+It is possible to implement custom handling for links with the
+#GtkLabel::activate-link signal and the gtk_label_get_current_uri() function.
+</para>
+
+</refsect2>
+
 <!-- ##### SECTION See_Also ##### -->
 <para>