gtkdoc-scan: recognise attribute-based deprecations

If we see something that looks like a macro for an attribute-based
deprecation (ie: some string matching /_DEPRECATED/) then mark the next
single declaration as <DEPRECATED/> in the -decl.txt. This avoids the
warning for missing deprecation guards.
Add Deprecated: tags to the doc-blobs of the existing tests to verify
that we don't get the warnings.

Fixes: #624001
Fix the hack in the previous commit

With 0.1 bump whenever we see _DEPRECATED, we can now enter
the nesting calculation with a counter value that is between
0 and 1. Therefore, change the checks to look for >=1 instead
of >0. This fixes parsing of an #ifdef in gtkstock.h.
Put examples in CDATA

https://bugzilla.gnome.org/show_bug.cgi?id=723118
Explicitly handle entities in |[

Now that we mark |[ blocks as CDATA that means legacy
usage that used entities to work around markup for
examples will be rendered incorrectly. So, we explicitly
convert them to text and print a deprecation warning.
Support |[<!-- language="C" -->

https://bugzilla.gnome.org/show_bug.cgi?id=723118