fix bug--do not update the files properly
fix bug--did not remove dir paths and suffix of files when delete files
const correctness
fix bug--did not clear the data member
fix bug--clear the file when add files or add directory
fix bug--do not show proper path of added files
fix bug--remove all did not clear some data
fix bug--may access non exist data
fix bug--add same files when add the files by add files and add directory
refine codes--some functions do not rely on function parameters again
update file names when add new files or directory
improve performance