C++ wrapper for gtksourceview, version 3.12.0.

Current dependencies:
- to build a library from tarball:
  * gtksourceview >= 3.12.0
  * glibmm >= 2.28.0
  * gtkmm >= 3.2.0
- to build a documentation:
  * same as above and tools required by mm-common, that are:
    - perl
    - doxygen
    - dot
    - xsltproc
- to build in maintainer mode:
  * same as above and tools used by maintainer:
    - git (to generate a ChangeLog)
    - mm-common >= 0.9.5
    - recent autotools because no deprecated macros are used.