Bug 622124 - implement flags for GSettings

Add a <flags> tag to the schema file format and a flags='' attribute to
go along with.  Add some extra test cases for those.

Add new g_settings_{get,set}_flags() calls and support binding to
GParamSpecFlags properties.  Add test cases.

gio/tests/schema-tests/flags-aliased-default.gschema.xml

@@ -0,0 +1,19 @@
+<schemalist>
+  <flags id='flags'>
+    <value nick='none' value='0'/>
+    <value nick='mourning' value='1'/>
+    <value nick='laughing' value='2'/>
+    <value nick='talking' value='4'/>
+    <value nick='walking' value='8'/>
+  </flags>
+
+  <schema id='xyz'>
+    <key name='abc' flags='flags'>
+      <aliases>
+        <alias value='speaking' target='talking'/>
+      </aliases>
+      <default>['speaking']</default>
+    </key>
+  </schema>
+</schemalist>
+