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

- macOS behaviour of setRealtime now matches other platforms

MR feedback

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.

Please see the forum post "Re-structuring of JUCE’s low-level Android code" and the BREAKING-CHANGES.txt for more information.