* Move include/* to src/FTGL/* so the files in there can directly be
referred to as "FTGL/*.h". This is convenient because they will be
installed in a similar location.
* Put a warning in each legacy public header to advise users to only include
the generic <FTGL/ftgl.h> header instead.
* End of the pImpl refactoring task started in [972]. FTGlyph was the last
class needing the change. As a consequence, FTGlyph is now also exported
in the library API, and so are FTBBox and FTPoint.