Fix columns settings restore on Sales app

Sales app was overriding calling get_columns again and thus getting them on the
default state (not the one with settings already restored). This solves the
issue by:

* Make SearchSlave._columns public (s/_columns/columns), so it can be accessed
  directly by .columns
* Use get_column_by_attribute instead of duplicating that code to find the
  right columns
* Remove a not used method 'SearchSlave.get_columns'. It's not called by anyone
  and doesn't make sense since SearchSlave receives columns on __init__
* Update the only other place other than Sales app accessing ._columns to
  access .columns instead

Change-Id: If47d3fe9444c36306cd3052fe00fa0ac87f1c078
Fixes: http://bugs.async.com.br/show_bug.cgi?id=5507
Merge "Fix columns settings restore on Sales app"