improve boxed types support
Submitted by Yeti
Boxed types are inspectable enough to appear as derived from GBoxed in the hierarchy so they should be there.
The current handling of MY_TYPE_FOO and my_foo_get_type() for boxed types is to dump them both in the section file with --rebuild-sections and to let people do anything if they don't rebuild sections. This means e.g in GLib MY_TYPE_FOO tends to be listed, wheres Gtk+ docs do not indicate whether a plain old data type is available as boxed or not.
MY_TYPE_FOO and my_foo_get_type() for boxed types probably should be put to
<SUBSECTION Standard> when generating the section file.
Once they are excluded it would be nice to insert `MyFoo implements/is derived from/provides/is available as/... GBoxed' somewhere automatically. Or added small hierarchy GBoxed -> MyFoo as is done for objects. Not sure how to implement either.
Boxed types can be defined in different place than the original type. E.g. G_TYPE_HASH_TABLE is defined in gobject because, obviously, it cannot go into glib. Not sure what to do in this case.