Refactor the model accesors

Refactor the accesors to behave like methods instead of properties.
This means all accesors will be prepended with the string
"get_" and will include a set of parenthesis.

Improve the ModeAttribute class to use the method's
short_description. This commit also adds support for a
new method .help_text attribute has been added.

Move accessors to their own module, named "methods.py".

Remove the PropertyHelper class as the accessors no longer
need it.

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

mayan/apps/cabinets/methods.py

@@ -0,0 +1,13 @@
+from __future__ import unicode_literals
+
+from django.apps import apps
+
+
+def method_get_document_cabinets(self):
+    DocumentCabinet = apps.get_models(
+        app_label='cabinets', model_name='DocumentCabinet'
+    )
+
+    return DocumentCabinet.objects.filter(documents=self).order_by(
+        'parent__label', 'label'
+    )