Create a BatchSelectionDialog

We have a new module, stoqlib.gui.dialogs.batchselectiondialog and inside it 2
dialogs for helping us with batch manipulation.

Actually, those 2 dialogs won't do anything. They will help us divide the
quantity into batches, for posterior stock increase/decrease. They both inherit
from a common class that is responsible for doing 90% of the logic there, like
appending new entries/spinbuttons and some basic validation on it.

The difference is that BatchIncreaseSelectionDialog will work with batch
numbers (since they probably doesn't exist yet) and
BatchDecreaseSelectionDailog works with StorableBatch objects and validate
their stock quantities.

Next commits will make use of those dialogs everywhere on Stoq!

Change-Id: I0a4a3dabe7e9708e395affba5bc364890f0a3b15
Merge "Create a BatchSelectionDialog"