filter start/due-before/after enhancement

additional flag for date filters of TODOLIST
attributes affected: startbefore, startafter, duebefore, dueafter
eg:
startafter:! list all tasks where startdate in todo is set
startafter:2014-10-30* list all tasks starting after given date or where no startdate in todo is set
user&date in todo when checked

add user and date information when checkbox is clicked