This fixes "Creation method of abstract class cannot be public" warnings
when compiling with recent valac.
Since g_mime_object_new[_type] are effectively static factory methods
rather than constructors, suppress their use as ctors and bind using
custom static methods.
Coverage decreased (-0.007%) to 64.488% when pulling 98b2db6853a0ae2ff52705a4c8d528cd09635279 on mjog:vapi-abstract-ctor into 3b71f3ff9e4d8882b685ba8071945f80833b62c5 on jstedfast:master.
This fixes "Creation method of abstract class cannot be public" warnings when compiling with recent valac.
Since
g_mime_object_new[_type]
are effectively static factory methods rather than constructors, suppress their use as ctors and bind using custom static methods.