Build options in multiple stages in template listbox helper

[ercanozkaya]

If a listbox is created out of thousands of entities they are all fetched at the same time resulting in memory limit errors.

Instead, fetch results 100 by 100 and gradually build the options array.

Note: this removes the 'unique' property of the listbox helper which made duplicate values appear only once in listboxes. However this was not a particularly helpful option since it could be problematic when you have more than one entity using the same title.

See joomlatools/joomlatools-framework#17