Improvement: factor window generation?

[mzed]

Both the fft class and the maxiGrains class do envelope generation, although one generates a whole buffer and the other returns values on the fly. It would be nice to factor that code into one envelope class. There should be an envelope type enum.

Also, an overloaded maxiFFT constructor that allows for different fft envelopes would be be nice. Currently, envelopes types are defined by a number. This should be a proper enum.