RTF comparison performance enhancement


We greatly improved the speed of the difference calculation of RTF fields which are shown in the item history.

The diff result of RTF fields is now calculated during log entry generation for field update and saved in DB. On histroy list display the diff result is then taken from that DB field instead of generating it every time. 

For existing log entries which doesn't have generated diff result yet, the link "show diff" is generated in history log entry. By clicking it, the diff is generated and saved.