redo feature and multi-level undo/redo (Issue #819)

thunar/thunar-action-manager.c

thunar/thunar-action-manager.h

thunar/thunar-application.c

thunar/thunar-application.h

thunar/thunar-clipboard-manager.c

thunar/thunar-dbus-service.c

thunar/thunar-dnd.c

thunar/thunar-enum-types.c

thunar/thunar-enum-types.h

thunar/thunar-io-jobs.c

thunar/thunar-job-operation.c

thunar/thunar-job-operation.h

thunar/thunar-menu.c

thunar/thunar-transfer-job.c