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/file_metadata/apps.py

@@ -121,16 +121,17 @@ class FileMetadataApp(MayanAppConfig):
             model=DocumentTypeSettings, related='document_type',
         )
 
-        SourceColumn(source=FileMetadataEntry, attribute='key')
-        SourceColumn(source=FileMetadataEntry, attribute='value')
+        SourceColumn(attribute='key', source=FileMetadataEntry)
+        SourceColumn(attribute='value', source=FileMetadataEntry)
         SourceColumn(
-            source=DocumentVersionDriverEntry, attribute='driver'
+            attribute='driver', source=DocumentVersionDriverEntry
         )
         SourceColumn(
-            source=DocumentVersionDriverEntry, attribute='driver__internal_name'
+            attribute='driver__internal_name',
+            source=DocumentVersionDriverEntry
         )
         SourceColumn(
-            source=DocumentVersionDriverEntry, attribute='get_attribute_count'
+            attribute='get_attribute_count', source=DocumentVersionDriverEntry
         )
 
         app.conf.task_queues.append(

mayan/apps/file_metadata/migrations/0002_auto_20181222_0931.py

@@ -0,0 +1,25 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.16 on 2018-12-22 09:31
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('file_metadata', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='filemetadataentry',
+            name='key',
+            field=models.CharField(db_index=True, help_text='Name of the file metadata entry.', max_length=255, verbose_name='Key'),
+        ),
+        migrations.AlterField(
+            model_name='filemetadataentry',
+            name='value',
+            field=models.CharField(db_index=True, help_text='Value of the file metadata entry.', max_length=255, verbose_name='Value'),
+        ),
+    ]