Several refactor actions and solutions for some Checkstyle issues

Added the createSortOptionPanel method. Removed the sortButtons list, since it was not really necessary.

Added some common code to the end of the createChartFrame method and renamed it to addChartFrame.
Added ZOOM_MIN_MAX_COMMAND constant for handling the zoom min max command (which solves some Checkstyle issues).
Added SORT_ORDER_ASCENDING_LABEL, SORT_ORDER_DESCENDING_LABEL and SORT_ORDER_COMPARE_LABEL constants for the sorting radio button labels.
Added SORT_COMMAND_PREFIX, SORT_COMMAND_SEPARATOR, SORT_ORDER_ASCENDING, SORT_ORDER_DESCENDING and SORT_ORDER_COMPARE constants for handling the sort commands.