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

- macOS behaviour of setRealtime now matches other platforms

MR feedback

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

In Windows DLL builds we inject operator new/delete definitions into
classes with a leak detector. This requires that all inheritance from
such classes must be public, and classes inheriting from multiple such
classes must disambiguate between the base's operators.

Before this commit an already started timer would wait until the
previously set tick time before applying the new one. This problem seems
to have affected the Windows implementation only.

This is a breaking change - see BREAKING-CHANGES.txt

This commit removes the various compiler-specific JUCE_DEPRECATED macros and replaces them with C++14's deprecated attribute. It also removes the JUCE_CATCH_DEPRECATED_CODE_MISUSE flag as we can rely on the override specifier catching usage of these old virtual methods, and tidies up the DOXYGEN preprocessor checks as they were inconsistent across the codebase.

This fixes warnings that are emitted when building with the `-analyze`
flag enabled.

Previously, if `stopTimer()` was called from within
`hiResTimerCallback()`, a call to `isTimerRunning()` immediately
following the call to `stopTimer()` would return true instead of false.

This patch fixes the issue, and adds some tests to verify the new
behaviour.