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.