GSettings: add <override>, tests, modify output

Add <override> tag, more tests, and actually output the results of these
new tags to the gschemas.compiled file.

gio/tests/schema-tests/extend-and-shadow.gschema.xml

@@ -0,0 +1,17 @@
+<schemalist>
+  <schema id='a'>
+    <key name='foo' type='s'>
+      <default>''</default>
+    </key>
+  </schema>
+
+  <schema id='b' extends='a'>
+    <key name='bar' type='s'>
+      <default>''</default>
+    </key>
+  </schema>
+
+  <schema id='c' extends='a'>
+    <key name='foo' type='s'/>
+  </schema>
+</schemalist>