IFileDialog::Show and CoUninitialize both seem to require the main
message loop to be active and running when they are called. If we block
the message thread while calling these functions, we may cause a
deadlock.

The destructor of the Win32NativeFileChooser was blocking the message
thread until the background thread exited, but the background thread was
unable to make progress while the message thread was blocked.

To work around this issue, we now pump the message thread in the
destructor of the Win32NativeFileChooser. If a dialog is currently
active, this should allow it to exit gracefully.

Note that we cannot use MessageManager::runDispatchLoopUntil here:
- MessageManager::runDispatchLoopUntil will not process any messages if
  the quit message has been received, which could lead to deadlocks if the
  FileChooser is destroyed after the quit message has been posted.
- This function isn't defined when JUCE_MODAL_LOOPS_PERMITTED is disabled.