This also fixes an issue where MIDI CI header data could contain spaces,
which is not allowed according to the spec.

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

This patch also makes Optional public to end users.

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.