SellableItemSlave: Allow to hide stock labels completely

* Add an attr that defines if the stock labels (stock/minimum_stock) are
  visible when a sellable with storable is selected.
* Update SaleReturnItemStep to use that attr and remove it from LoanItemStep
  since on a Loan it make sense to see the total stock for that sellable.
* Put manager inside __init__ since it's used in the object, not in the class.

Change-Id: Ib0a9f8ef0e74a9e5a6e2e3b7edc0a9748a87a305
Make BatchSelectionDialog public

* Make BatchSelectionDialog public, making it a valid choice when something is
  being selected for neither a decrease or a increase, and also make it
  possible to extend it directly from the base (instead of extending by it's
  2 children).
* Add an attr that will dictate if the existing batches objectlist is
  visible or not.
* Add an attr that will dictate if pressing enter on an entry will confirm the
  dialog
* Add an attr that will dictate if we can indicate a batch with a quantity of 0
* Make all class attrs lower_case. Being ALL_CAPS could be confused as that
  attr was a constant (and a constant shouldn't change value, while those can
  "change" on subclasses.

Change-Id: I9192aad8db327de1df7952ad061e6ea82c71f3ae
Merge changes I9192aad8,Ib0a9f8ef

* changes:
  Make BatchSelectionDialog public
  SellableItemSlave: Allow to hide stock labels completely