This allows newer versions of sonic to be used and to keep
up-to-date with bug fixes provided by the system.

This removes the GUI part of espeakedit, keeping the part that is
responsible for compiling the voice and intonation data. The
motivation for this is:

  1.  the voice and intonation data compilation will move into
      libespeak-ng after removing the wxWidgets logic and converting
      the code to C;

  2.  the espeakedit code is tightly coupled to the internals of
      libespeak-ng at a source code level, not a binary/API level,
      making the code harder to maintain;

  3.  a new GUI will be written in Qt that better supports creating
      and editing voices and languages.

This moves the libespeak source (*.cpp) files to src/libespeak-ng
and renames the C files (libespeak, espeak, speak) so they have
a .c file extension.

This is based on a patch by Thomas Petazzoni.