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.

docs/reference/gio/gio-sections.txt

@@ -2168,6 +2168,8 @@ g_settings_get_strv
 g_settings_set_strv
 g_settings_get_enum
 g_settings_set_enum
+g_settings_get_flags
+g_settings_set_flags
 
 <SUBSECTION MappedGet>
 GSettingsGetMapping