Autosave map functionality (#995)

Added autosaving of the current map. Autosaving happens in intervals when the map is changed. It will not overwrite the current map, but rather create new files, just like backups. Autosaving interval and number of files can be configured in the "Recovery" tab of the preferences. Autosaving can also be disabled there (not recommended). Autosaves will not have their nodes built for performance reason.