C++ wrapper for vte, version 0.20.3.

Current dependencies:
- to build a library:
  * vte >= 0.20.5
  * gtkmm >= 2.14
  * glibmm >= 2.18
- to build a documentation:
  * doxygen
  * xsltproc
  * gtkmm_doxygen_tags, glibmm_doxygen_tags and pangomm_doxygen_tags for
    proper xrefs. (see TAGFILES in docs/reference/Doxyfile)

