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.
We only ever do the enum mapping for the property binding in the case
that a GParamSpecEnum exists and in that case the class is already
referenced by the GParamSpec. Use peek instead of ref/unref and add a
clarifying note.
