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/metadata/widgets.py

@@ -1,17 +1,26 @@
 from __future__ import unicode_literals
 
 from django.utils.html import format_html_join
+from django.utils.translation import ugettext_lazy as _
 
 
-def get_metadata_string(document):
+def widget_get_metadata_string(context, attribute=None):
     """
     Return a formated representation of a document's metadata values
     """
+    obj = context['object']
+
+    if attribute:
+        obj = getattr(context['object'], attribute)
+
     return format_html_join(
         '\n', '<div class="metadata-display"><b>{}: </b><span data-metadata-type="{}" data-pk="{}">{}</span></div>',
         (
             (
                 document_metadata.metadata_type, document_metadata.metadata_type_id, document_metadata.id, document_metadata.value
-            ) for document_metadata in document.metadata.all()
+            ) for document_metadata in obj.metadata.all()
         )
     )
+
+
+widget_get_metadata_string.short_description = _('Metadata')