Added GTK_BUILDER_ERROR_INVALID_PROPERTY and GTK_BUILDER_ERROR_INVALID_SIGNAL
error codes

ObjectInfo: Use a GType instead of a char * for the class name.
PropertyInfo: Use a GParamSpec instead of a char * for the property name.
SignalInfo: Use signal id and detail quark instead of a detailed signal name string.

This not only save us a few malloc in each case but lets us simplify the code
and report unknown properties and signals as a parsing error instead of just
printing a warning.