Some hosts (such as Reaper) weren't creating an instance of exactly
NSOpenPanel or NSSavePanel, which meant that the call to
`getSelectedPaths` was always returning an empty array.

Now, rather than trying to cast the sender to an appropriate type, we
assume that the sender is always the same panel that we opened earlier,
so that we can just query the panel directly for its selection.

Previously, when a file dialog was launched synchronously, the 'dummy' modal
component was preventing user input events from reaching the preview component.

Now, we explicitly allow input events to reach the file preview component, if
it exists.

If the preview is not removed from the desktop, the FileChooser will try to reuse
the same ComponentPeer the next time the preview component is used.

Previously, when the FileChooser::Native instance entered the
modal state, it was preventing the inner FilePreviewComponent from
receiving events.

The Win32NativeFileChooser was taking ownership of itself
in its `Thread::run` implementation. This meant that sometimes
the destructor of the file chooser thread would execute directly
in `Thread::run`.

Now, we explicitly transfer ownership into a function object which
will run asynchronously on the main thread. This way, the file chooser
thread will be stopped on the main thread.