Code changes for compatibility with JUCE 6.x

- Replace all juce::CriticalSection with std::recursive_mutex
- Replace all juce::AudioSampleBuffer with juce::AudioBuffer<float>
- Eliminate implicit reliance on 'using namespace juce;'
- Replace OpenShotAudio.h includes with targeted juce modules

src/ReaderBase.cpp

@@ -20,6 +20,7 @@
 
 #include "Json.h"
 
+
 using namespace openshot;
 
 /// Constructor for the base reader, where many things are initialized.