Native Instrument VST3s (I tested Kontakt and Supercharger) crash when
the bundleEntry function is called from a background thread on macOS.
This change disables scanning for VST3 plugins on a background thread
using the PluginListComponent, to allow loading these plugins in the
AudioPluginHost.

I can't find any "official" word on whether the bundleEntry and
bundleExit functions should be guaranteed to be made from the main
thread. However, the VST3PluginTestHost app seems to call these
functions exclusively from the main thread.

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

This fixes warnings that are emitted when building with the `-analyze`
flag enabled.