Add widget support to SourceColumn

Allow passing a widget class to SourceColumn. This makes
using lambdas to render model column unnecesary and are
mostly removed too.

Signed-off-by: Roberto Rosario <Roberto.Rosario@mayan-edms.com>

mayan/apps/dashboards/apps.py

@@ -1,6 +1,5 @@
 from __future__ import unicode_literals
 
-from django.apps import apps
 from django.utils.translation import ugettext_lazy as _
 
 from mayan.apps.common import MayanAppConfig