Use generic method of fetching typed value from pointer
Provide GObject.signal_connect_after
Remove a TODO

Auto-converting between quarks and strings cannot be done generally,
since the introspection data shows all quarks simply as uint32.
Provide GObject::Object#signal_connect_after
Handle setting enum GValues
Handle setting enum GValues as integers
Handle enum GValues and properties [merges 'issue-47-set-enum-properties']