While the affected callbacks are cancelled before the referenced
state is deleted, we have had user reports that they can still be
accessed by the cancelled callbacks causing crashes. After only
finding warnings that WinRT AsyncCallback cancellation is not a
guaranteed thing, we saw it best to wrap the pointers.

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.