gtkdoc-mkdb: Add support for (nullable) and (optional) annotations

These deprecate and replace (allow-none), which was ill-defined for
(out) parameters.

This includes some updated unit tests.

Including a wording contribution from Evan Nemerson
<evan@coeus-group.com>.

https://bugzilla.gnome.org/show_bug.cgi?id=730777
docs: add info for creating better tarballs
tests: Add a macro with DEPRECATED in its name

It should not be marked as deprecated itself.

https://bugzilla.gnome.org/show_bug.cgi?id=731417
scan: Ignore macro definitions with DEPRECATED in their name

These should not be automatically deprecated.

https://bugzilla.gnome.org/show_bug.cgi?id=731417
tests: Add an enumeration value with DEPRECATED in its name

This should not mean that the enumeration is deprecated.

https://bugzilla.gnome.org/show_bug.cgi?id=730658
scan: Ignore enumeration fields with DEPRECATED in their name

GParamFlags should not be declared deprecated just because it has a
value called G_PARAM_DEPRECATED.

https://bugzilla.gnome.org/show_bug.cgi?id=730658
mkdb: drop legacy 'return' description handling

We don't recognize sentences starting with 'Returns' as return docs anymore.
This was ambigous and we printed a warning for this since years.

FIxes #678094
scanobj: remove gtk_object specific scanner

When building docs for gtk+-1.X one will need to use an older gtk-doc version.
release: update news and date in manual
scanobj: remove Makefile target
release: bump versions and back to development
Updated Brazilian Portuguese translation
Updated Spanish translation