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/checkouts/methods.py

@@ -0,0 +1,33 @@
+from __future__ import unicode_literals
+
+from django.apps import apps
+
+
+def method_check_in(self, user=None):
+    DocumentCheckout = apps.get_model(
+        app_label='checkouts', model_name='DocumentCheckout'
+    )
+    return DocumentCheckout.objects.check_in_document(
+        document=self, user=user
+    )
+
+
+def method_get_checkout_info(self):
+    DocumentCheckout = apps.get_model(
+        app_label='checkouts', model_name='DocumentCheckout'
+    )
+    return DocumentCheckout.objects.get_document_checkout_info(document=self)
+
+
+def method_get_checkout_state(self):
+    DocumentCheckout = apps.get_model(
+        app_label='checkouts', model_name='DocumentCheckout'
+    )
+    return DocumentCheckout.objects.get_document_checkout_state(document=self)
+
+
+def method_is_checked_out(self):
+    DocumentCheckout = apps.get_model(
+        app_label='checkouts', model_name='DocumentCheckout'
+    )
+    return DocumentCheckout.objects.is_document_checked_out(document=self)