Make callback function extendable
Don't export internal symbols
gi: install public header and .pc
gi: add missing "_callback"
gi: add missing "_func"
gi: remove needless "_callback"
gi: reduce needless get_g_type()

It fails for GI_INFO_TYPE_CALLBACK.
gstreamer-gi: add for testing GObject Introspection based binding
gstreamer-gi: use explisit BOXED2RVAL for GstMiniObject

GstMiniObject can't use GOBJ2RVAL. :<
gi: support not GObject based enum