giscanner: deprecate top level g-i specific GTK-Doc tags.

Rationale
---------

The first wave of improvements on AnnotationParser intended
to fix bug http://bugzilla.gnome.org/show_bug.cgi?id=672254
(multiline descriptions on tags not working correctly, polluting
the comment block description instead) had an unforeseen side
effect reported in bug http://bugzilla.gnome.org/show_bug.cgi?id=674658.

Analyzing this problem we found that even though comment blocks
having g-i specific tags before the comment block description
are technically malformed, the old annotationparser code happily
accepted these without complaint. During this analysis, a secondary
bug report (http://bugzilla.gnome.org/show_bug.cgi?id=676133)
was filed with the GTK-Doc project because it was found that the
same malformed tags did not result in the expected html output.

After discussing this with the GTK-Doc maintainers, we learned
that our g-i specific top level tags should never have existed
in the first place. The prefered notation for annotations that
apply to the identifier should be written on the identifier line,
for example like what we already do with (skip).

As a result, this patch deprecates g-i specific top level tags and
implements them as annotations on the identifier instead and at
the same time adds support for malformed comment blocks still
using g-i specific top level tags.

This means that all annotated code "out there" will continue to work
just fine with this version of g-i, but when a developer decides to
fix deprecation warnings in his/her comment blocks, the dependency
on g-i needs to be raised to a version that contains at least this
patch.

Overview of changes
-------------------

1) giscanner/annotationparser.py and giscanner/maintransformer.py
   have been adjusted to deprecate g-i specific top level tags
   and now implement these in terms of annotations on the identifier.
   The Attributes: tag and it's replacement (attributes) annotation
   continue to be broken (regression introduced in commit
   182fdfec261f36fa095484352b9b834861036907), which will be
   addressed in a followup patch.

2) AnnotationParser().parse_options() has learned to correctly parse
   multiple closing parentheses, which is for example required for the
   new syntax in tests/scanner/annotationparser/gi/annotation_type.xml:
   * AnnotationObject:hash-table: (type GLib.HashTable(utf8,gint8))

3) comment blocks in tests/*.c and tests/*.h had to be adjusted as
   these tests are not allowed to emit warnings. As a result, these
   test cases now no longer use g-i specific top level tags but
   annotations on the identifier instead.

https://bugzilla.gnome.org/show_bug.cgi?id=688897
https://bugzilla.gnome.org/show_bug.cgi?id=676133
https://bugzilla.gnome.org/show_bug.cgi?id=674658
tests: validate GTK-Doc test files

This is more of a handy developer tool than something
that is always expected to be executed as part of the
test suite, so we don't add a hard dependency on xmllint.
Add a tool fixing GTK-Doc comment blocks in .c and .h files

Just a standalone script for now, should probably eventually
go into g-ir-doctool or g-ir-annotation-tool...
docs: document some annotationparser.py hacking hints
Add giscanner package documentation

Using Sphinx [1] markup for now. We eventually might want
to add GTK-Doc support for python and dogfood g-ir-doctool
on this package.

[1] De facto standard Python documentaton tool: http://sphinx-doc.org
[WIP] docs: improve layout
!!! for testing only !!!
g-ir-compiler: remove references to --code, remove --no-init options

The "--code" option was removed years ago in ac81f3e8c5f1c380d16677232d67466e739da283
so remove references to it from README and g-ir-compiler(1)

Remove the "--no-init" option from g-ir-compiler and g-ir-compiler(1)
as it was documented to "can only be used if --code is also specified",
so no reason to keep it around.
build: actually require libffi >= 3.0.0

as documented in README...
README: fix the Tools section a bit