fft: Moved all window building functions into C++.

Remove window.py. All windows built using C++ functions. Exported into Python in same module (from gnuradio.fft import window).

Removed all window building work in firdes, too. firdes.window now makes a direct call to fft.window.build with same parameters.

Added documentation for window functions, including references to where to find coefficients and equations for (most of) the windows.

All changes should not affect existing code.

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