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.

If an aiff file advertied a large number of metadata keys, the
StringPairArray::set calls resulted in quadratic complexity.

We now read keys into a std::map instead, as it enables much faster
lookup, and then convert back to a StringPairArray at the last possible
moment.