Enhancements and fixes
- Satellite and Star tracker plugins: dark theme option for charts as default
- Spectrum: increase maximum FFT size to 32k
- Change delete order of channel and GUI. Fixes #860
- APT demod: moved processPixels process to a separate thread. Implements #865
- APT demod optimization: do a full image process only every 20 lines
- DAB/DAB+ demod: added images in GUI and various fixes. PR #866
- DAB/DAB+ demod: fixed right-click audio selection. PR #856 fixes #853
- SampleSinkFIFO: read/write return immediately if size is zero. Fixes #868