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.