Added MainWindowGUIObject
Refactoring: Pushed parse_file_for_import() up from Document to MainWindow

This is to avoid the stupid csv_options_needed notification.

Qt UI has been adapted, Cocoa still needed.
Refactoring: Pushed load_parsed_file_for_import() from Document to MainWindow

This is to avoid the stupid file_loaded_for_import notification.

Qt UI has been adapted, Cocoa still needed.
Refactoring: Import "loader" is now hosted in MainWindow rather than Document
Fixed build on OS X
Refactoring: Adapted Cocoa layer to latest ImportWindow changes
Removed spurious imports in mg_cocoa