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/navigation/exceptions.py

@@ -0,0 +1,5 @@
+from __future__ import unicode_literals
+
+
+class NavigationError(Exception):
+    """Base navigation app exception"""