build: Say good-bye to intltool

intltool is unmaintained nowadays while upstream gettext gained
support for everything we used it for, so make the switch.

https://bugzilla.gnome.org/show_bug.cgi?id=769077

extensions/example/org.gnome.shell.extensions.example.gschema.xml

@@ -0,0 +1,9 @@
+<schemalist gettext-domain="gnome-shell-extensions">
+  <schema id="org.gnome.shell.extensions.example" path="/org/gnome/shell/extensions/example/">
+    <key name="hello-text" type="s">
+      <default>''</default>
+      <summary>Alternative greeting text.</summary>
+      <description>If not empty, it contains the text that will be shown when clicking on the panel.</description>
+    </key>
+  </schema>
+</schemalist>