#21 minor refactoring

- warn before quitting
- open files
#21 minor reformatting
#21 refactor filter items operation

- bugfix: resume file items operations
 #21 wait when  invalidating cache handler for file browser (race condition...)
#21 remove unnec' comments
#21 delete measure time
#21 make properties nonatomic
close #21: Merge branch 'develop'