LatexilaSynctex (not finished)

It is needed for the build tools for opening a PDF file with synctex
support with evince. Since I don't want to use the Vala code in C, a
rewrite of synctex.vala in C was needed.

synctex.vala uses the synchronous API, which is bad. The rewrite in C
uses the asynchronous API (it's better).