G-ir fixes and annotations

[Ankit Vani]

I haven't had much time lately to work on the annotations (the only incomplete
thing, which also turns out to be a bit tedious). But I'm keeping the
gobj.plugins branch up to date with the changes in main, for when I do get time.
If there is anybody who could assist with these, please do so. Although I'm
generating the gir warnings from the gobj.plugins branch that point me to what
annotations or docs are missing, and what types need to be made GBoxed so that
introspection can be used with them, I've been doing these doc updates and
boxing directly in main.

If there is anyone who would be willing to help out, this is generally what
needs to be done:
- Add missing annotations in the documentation.
- Convert bare structs into GBoxed so that introspection can work with those
  types.
- Add missing documentation (if needed).

When being used from plugins in other languages, boxed structures are often
copied by value. For non-GObject/bare struct types that we do not want an
instance copied for, we can add simple ref counting to be done from the GBoxed's
copy and free callbacks. Or if there's a better way, please let me know.

I hoped to have more free time by today but things have kept piling up.
Hopefully I'll be able to offer more by next month.

Ankit