Handle application and window closing (Synergy, cb9a22c)

Special handling for application and window closing if there are unsaved changes. To get this to actually work I also had to fix a bug in the loading code: we were instantiating an instance of WOPreferencePane rather than the subclass as specified by the NSPrincipalClass key in the bundle info property list.

Signed-off-by: Greg Hurrell <greg@hurrell.net>