* 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.
* Move FTGlyph, FTFont and FTLayout classes and their derivatives into
separate subdirectories of src/ to avoid cluttering src/ with too many
files. The Visual Studio solution still needs an update.