This reverts commit 6d38d7eb33.

Previously, when saving a temporary project, two native dialogs would be
created. The first was very short-lived, as it was automatically
dismissed by the second dialog. On older macOS versions, this appeared
to work correctly (the first dialog was so short-lived that it did not
appear to the user). However, on macOS 12.0, the second dialog fails to
display altogether.

Displaying two file chooser dialogs feels like a programming error, so
the code has been reorganised a bit to avoid this case.

- 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

When loading exporter icons on a background save thread we need to take the MessageManagerLock as the image file may be an SVG which has a message thread dependency in the Drawable parsing code

See BREAKING-CHANGES.txt for more details.

Fixes an issue where module sources with names including `_Standalone_`, `_AU_`
etc. were added to the target for that plugin format, instead of being added to
the shared code target.