This was part of espeakedit, and used wxWidgets to draw the
envelope graphs.

This removes the implementation of those functions from espeak-ng
and use the versions in libespeak-ng.

NOTE: This is needed to cross-compile to Windows.

This creates an fread compatible API on wxInputStream objects and
uses that API in LoadSpectSeq. This is to help the transition from
the wxWidgets wxInputStream API to the C FILE API.

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.