Windows: Fix/Improve closing of native file dialogs.

IFileDialog::close() only works from callbacks. Try to find
the dialog window and send it a WM_CLOSE in addition.

Change-Id: Id0f89f8781564e19e4763d43a71df55d5299fb35
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>