Implement a preloading generator for ItemPages

Since it becomes near impossible to find which specific
item lines up with which original page object, we just
create new objects and yield those.

WikibasePage.__defined_by had to be renamed so it is
no longer a private method.
Change-Id: I05917eb982f41dcdc92d8e45292ba6c27eac47f7
Merge "Implement a preloading generator for ItemPages"