Rework Task css class helper

I've moved the decision which icon should be displayed, directly to the SQLTask class, instead of using an helper handling all different objects(brains, Tasks, SqlTasks etc.). According to this changes some other adjustments are necessary: 
 - Fixed #363 
 - Change css_getter in task byline
 - Reworked `Task.get_link` use css_class getter of task, instead of passing it by.
 - Dropped `overdue_date_helper` and moved it tho the SQLTask class as `deadline_label` method.
 - Reworked and cleaned  up `sqltablelisting` textfilter functionality.


@deiferni