Remove using namespace juce and juce module headers from generated JuceHeader.h

[rsjbailey]

Currently, the whole juce namespace is imported into files which make use of the framework.
This is generally not great practice, and has begun to make the project sensitive to #include ordering, as there's a potential conflict with the Point struct in the macOS CoreFoundation framework and the juce::Point class.

We should

• Remove using namespace juce; from JuceHeader.h.in
• Go through all headers that #include "JuceHeader_Wrapper.h"
• Explicitly qualify the juce classes used in our interfaces
• For each .cpp file, decide whether it's appropriate to import the juce namespace or whether we should selectively import names from that namespace.

The same header has #include directives for every juce module in the project, which is likely not optimal for compilation times as not all classes use all modules.

We could probably improve this fairly rapidly by

• Copy/Pasting these #includes into each class header
• Removing them from JuceHeader.h.in
• Running include-what-you-use

I've never actually tried include-what-you-use and I'm not sure how clever it is, maybe you can skip step one or perhaps there's an additional step required to move some includes into the .cpp file when possible.

It probably makes sense to do the namespace qualification before the second stage, as if we already have ambiguous lookup I'd imagine include-what-you-use is going to struggle.