The old API only allowed cancelling property "get" inquiries and
subscription updates. However, there are use-cases for cancelling other
requests too. e.g. switching between views in a JUCE app might mean that
it's no longer necessary to subscribe to a particular property.

Cancelling subscriptions ends up being quite involved. Different
handling is needed depending on whether the subscription is cancelled
before or after the responder replies to the initial request.
In addition, the responder may ask the initiator to retry a subscription
begin request.

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.