- Also refactored internals to use weak/shared pointers

This allows real-time threads to join an audio workgroup on Apple platforms.

- added unit tests
- best performance timers used for each platform
- fixed an issue in which timer callbacks could drift

- Created a new detail namespace
- Moved shared module implementation details into the detail namespace
- Split dependencies so source files only rely on details in the detail namespace
- Removed all code from the juce_gui_basics.cpp file

This includes Carbon support

This restores the functionality of JUCE_COREGRAPHICS_RENDER_WITH_MULTIPLE_PAINT_CALLS.
Using this preprocessor flag may avoid Core Graphics rendering much larger regions than
necessary, but the small regions that are rendered will likely be rendered slower.
Whether using this flag improves or degrades the performance of your rendering overall
will be specific to each application.

Previously enabling JUCE_COREGRAPHICS_RENDER_WITH_MULTIPLE_PAINT_CALLS was ineffective
from versions of macOS around 10.13, but enabling it didn't have any negative impact on
performance. Now enabling JUCE_COREGRAPHICS_RENDER_WITH_MULTIPLE_PAINT_CALLS may result
in slower rendering.