Add attribute deleted to all objects, for held instances.

If an instance is being held in two places, and then one of them
calls delete(), the other can check deleted before doing anything with
the object.

etiquette/objects.py

@@ -33,6 +33,7 @@ class ObjectBase:
     def __init__(self, photodb):
         super().__init__()
         self.photodb = photodb
+        self.deleted = False
 
     def __eq__(self, other):
         return (
@@ -157,6 +158,7 @@ class GroupableMixin:
         # issues of recursion.
         self.photodb.sql_delete(table=self.group_table, pairs={'memberid': self.id})
         self._uncache()
+        self.deleted = True
 
     def get_children(self):
         child_rows = self.photodb.sql_select(
@@ -372,6 +374,7 @@ class Album(ObjectBase, GroupableMixin):
         self.photodb.sql_delete(table='album_photo_rel', pairs={'albumid': self.id})
         self.photodb.sql_delete(table='albums', pairs={'id': self.id})
         self._uncache()
+        self.deleted = True
 
     @property
     def display_name(self):
@@ -596,6 +599,7 @@ class Bookmark(ObjectBase):
     def delete(self):
         self.photodb.sql_delete(table='bookmarks', pairs={'id': self.id})
         self._uncache()
+        self.deleted = True
 
     @decorators.required_feature('bookmark.edit')
     @decorators.transaction
@@ -775,6 +779,7 @@ class Photo(ObjectBase):
             queue_action = {'action': action, 'args': [path]}
             self.photodb.on_commit_queue.append(queue_action)
         self._uncache()
+        self.deleted = True
 
     @property
     def duration_string(self):
@@ -1365,6 +1370,7 @@ class Tag(ObjectBase, GroupableMixin):
         self.photodb.sql_delete(table='tags', pairs={'id': self.id})
         self.photodb.caches['tag_exports'].clear()
         self._uncache()
+        self.deleted = True
 
     @decorators.required_feature('tag.edit')
     @decorators.transaction