Create liblatexila with empty GObject class

The liblatexila is not a real library like GTK+, it is just an internal
library. The code will be written in C in this directory. The Vala code
in the parent directory can use the liblatexila, but I'll try to not do
the reverse (use the Vala code in liblatexila) so the liblatexila is
self-contained and could be reused easily by other projects (a gedit
plugin for example).

Why the C language? Well, I now prefer the C language. The Vala syntax
is nice, but it adds another layer. Another layer of potential bugs.
Another layer of maintenance. Another layer for the API documentation.
The GNOME documentation with GTK-Doc is primarily written for the C
language. And the verbosity of the C language has advantages
("greppable" code, code more self-documented).
build: use GNOME_COMPILE_WARNINGS

For the C code in the liblatexila it's important.
Update the hacking file
LatexilaPostProcessor base class
LatexilaPostProcessorAllOutput
Add 'priv' in the object structs

I don't like using all the time get_instance_private().
LatexilaBuildTools classes

And add a latexila-types.h for convenience.
Add GObject introspection support

It will be used to generate the .vapi for the liblatexila.
Generate latexila.vapi with GObject Introspection
latexila.h to ease introspection support
build tools: add missing GI annotations
Generate internal GTK-Doc documentation
Doc fixes
BuildTool and BuildJob classes

And add enum types.
BuildToolsPersonal: small bug fix

If save() is called a second time while the first call is still running
(it is an asynchronous function), the second call is just ignored.
First unit test in latexila, yay! (better late than never)

It tests the build tools loading from an XML file.
Add code coverage support

New make target:
$ make check-code-coverage

To have unit tests code coverage with an HTML report with colors etc.
Disable GTK-Doc tests

It is almost impossible to pass those tests.
Update git.mk for the code coverage files
LatexilaBuildView

Almost finished, there is no icons, and the path doesn't have a tilde
for the home directory (it is not the end of the world for a first
iteration).