option to remember sorting and filtering state

sorting and filtering methods are converted to a constant integer
before saving since the ids are probably not very persistent.

a check is added to determine checked state of the menuitems
each time the menu is opened to keep consistency between the
activities.
revise title and description of wakelock preference

as suggested by issacluz - the technical term is moved from title
to description.
show notifications with a static method
change method of getting id for notifications

System is faster than using Calendar
more detailed feedback when deleting backups of uninstalled apps

as suggested in issue #49
(https://github.com/jensstein/oandbackup/issues/49)
show a list of backups to be deleted, make a notification
when done and show a toast if there is nothing to delete