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.

Android devices won't necessarily be able to load MP3 files, so that
option has been removed from the file chooser. FLAC files should be
supported, but were missing from the MIME table.

The demo no longer tries to load files into the thumbnail view if they
previously failed to load into the transport.

This patch also updates the MidiDemo to automatically refresh the device
lists when the set of available devices changes.

This is a breaking change - see BREAKING-CHANGES.txt

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.

- Add AlertWindow::show() and showAsync() methods that take a MessageBoxOptions argument
  - Add NativeMessageBox::show() and showAsync() methods that take a MessageBoxOptions argument
  - Update the DialogsDemo to demonstrate the new methods
  - Deprecate AlertWindow::showNativeDialogBox() in favour of the NativeMessageBox methods
  - Pass button strings specified in MesssageBoxOptions to native dialog boxes correctly
  - Use modern TaskDialog on Windows for the native dialog box where available

See BREAKING-CHANGES.txt for more details.