add features:
1. if the record is modified ,the program will set the state to
"unsaved"
2. when program exits or open record file, the program will check
whether the state is "unsaved". If the state is saved, a dialog will be
pop up to ask whether the user will save it.
3. when the program exits, it will write the name of the opened record
file into an tmp file. when the pogram stats, it will read the tmp file
first, and try to reopen the most recently opend record file.