Merging a plist which contained UIBackgroundModes or
UISupportedInterfaceOrientations keys could result in these keys being
duplicated in the generated plist.

This patch will avoid adding a new array if the array's key already
exists in the plist.

Without the BOM, VS assumes UTF-8 strings (subproject file names etc.)
are ansi-encoded.

AU and AUv3 plugins only support version numbers with up to 3 parts.

- 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