The current minimum-supported Xcode (10.1) includes the iOS 12.1 SDK, so
APIs from iOS 10 will always be available.

Changed the lifetime of the default MIDI oputput.
Removed the need for Timer workaround in AudioDeviceSelectorComponent.

f1b6bbc921 erroneously replaced

    callback = newCallback;
    previousCallback = callback;

with

    previousCallback = std::exchange (callback, newCallback);

It looks like previousCallback is intended to hold the last active
callback, so the previous version was correct after all.

This fixes an issue where specifying an audio device type before calling
initialise could cause a different device to become active, even if the
requested device type had usable devices.

With this change in place, a new device type will only be selected if
the current device type has no devices.

The AudioPlaybackDemo was previously triggering thread sanitizer
warnings when starting playback.

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.