Initial commit for cross-platform (Mono) support.

Needed to remove the P/Invoke because there's no kernel32.dll on Mono.
But unfortunately, the application crashes at the export functionality's
pull notification (FileSystemWatcher.Changed) event.
Fixed GDI+ null reference on application close
Fixed Mono crashing on multicast delegates for FileWatch
Fixed Main disabling itself on Mono and not using platform-specific path separator
Added properly handling file overwrite prompt for exports
Fixed collection modifying itself from a foreach