f4exb/sdrangel v4.14.6

Spectrum analyzer code reorganization and fixes

on GitHub

Spectrum analyzer code reorganization and fixes

Code reorganization to isolate the spectrum analysis code (SpectrumVis class) from GUI opening the way to remote spectrum functionality already available on the v5 branch.

Also fixed following bugs:

• FFTFactory: add mutex for thread safety. Implements issue #514
• SpectrumVis: release previous FFT engine allocation from FFT factory only if allocated once. Fixes issue #515