fft: optimization; bug fixes for kaiser window and added windows to win_type.

filter: fixed width_factor to include new window types and add protection around array.
qtgui: fix for issue #615.
Merge branch 'maint'
qtgui: fft/waterfall plot parameters are checkable and
filter: remove use of magic numbers for calculating number of taps in firdes filters.

New way uses known max attenuation values for the windows to estimate the number of taps used in a filter. This produces smaller filters but needed QA code with pre-defined values updated.