SSB demod and Main Window enhancements

SSB demod

• Added control of FFT filter window and a bank of filters with the possibility to switch between 10 filters. Implements #1224

Main Window

• If there is no default configuration or this default configuration is empty (contains no workspaces) it will create an empty workspace. Part of #1250
• Save and restore auto-stack option in workspaces in configurations (including default). Part of #1250
• Warn if OpenGL is < 2.1, rather than 3.0, as it runs fine with 2.1. PR #1249

Other changes and fixes

• Ability to build a package for MacOS. PR #1249
• Fixed ButtonSwitch background color with stylesheets
• Added 3 term Blackman and 7 term Blackman-Harris FFT windows