WIP: move saleprice from stock to stocktype
Bugfix: check focus properly
Make popupfield and listfield relate to models only

popupfield and listfield were only used for choosing database models.
Make this explicit, and arrange that the models are always added to the
current database session before being passed anywhere.
WIP: move of saleprice column from stock to stocktypes
UI fixes for deliveries

New "positive integer" field validation type

Be careful when refreshing models

Bugfix for scrollables with zero-length display lists
Finish update of delivery UI

Now copes with saleprice being attached to StockType not StockItem

Allows multiple units to be entered in one go and divides cost price
amongst them