Invalid use of g_boxed functions in SDPMessage

As discovered in gst-plugins-base!999 (comment 746236) it turns out that SDPMessage uses boxed copy/free even though specific functions exist (gst_sdp_message_copy/free). It seems invalid to copy and free this record verbatim anyway.

However, that's what was generated by gir previously before this got migrated over to a manual file.

@slomo Suggested these copy functions might have been introduced later, with which he might mean handling of those in gir? The original bindings with g_boxed_copy were introduced about 2 years ago but copy and free exist for about 7 years.

This likely has to be fixed but follows the question: is the heuristic of having just get_type to get AutoBoxed correct? gst-plugins-base!999 (merged) adds get_type for a bunch of minobjects, turning these (incorrectly) into boxed types. Suddenly all sense of lifetime, ownership and cleanup is gone.

Finally, one actionable change is to remove the use_boxed_types property from gir which is:

• Unused (after above linked patch);
• Requires get_type (and is implied if that but not the other functions are available);
• Is hence only useful to force usage of g_boxed_copy/free even if (un)ref or copy/free are available: the first paragraph of this issue asks whether that's completely invalid...