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/enum-with-repeated-value.gschema.xml

@@ -0,0 +1,10 @@
+<schemalist>
+  <enum id='org.gtk.test.MyEnum'>
+    <value nick='nospam' value='0'/>
+    <value nick='spam' value='1'/>
+    <value nick='ham' value='2'/>
+    <value nick='eggs' value='3'/>
+    <value nick='bangers' value='4'/>
+    <value nick='mash' value='1'/>
+  </enum>
+</schemalist>