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/smart_settings/tests/test_classes.py

@@ -37,4 +37,3 @@ class ClassesTestCase(BaseTestCase):
         setting_value = setting_paginate_by.value
         os.environ.pop('MAYAN_{}'.format(TEST_SETTING_NAME))
         self.assertEqual(setting_value, TEST_SETTING_VALUE)
-