Add reworked `deadline_label` implementation to Task class.
Reworked sqltablelisting textfilter functionality.

Define searchable_columns in the BaseTableSource instead
of guessing the columns on the configured helper.
Dropped `overdue_date_helper` and replaced with lambda function.
Added taskquery option `by_brain`.
Reworked _get_task_css_class helper, it uses now task objects `get_css_class` method.
Fixed css_class getter in the task byline.
Reworked `Task.get_link` use css_class getter of task, instead of passing it by.
Merge pull request #474 from 4teamwork/pg_css_classs

Rework Task css class helper