Major changes. ints to guids etc.

- Categories now has unique identifiers instead of integer
Tables and classes changes: Makers, Products, PrductTypes
- Vendors now has unique identifiers instead of integer
Tables and classes changes: DiscountCards, Statistics
- A new digest form CategoriesListForm with the product categories and
types lists
- A few small bug fixes
- A column 'Payed' of Purchases.Receipts has been renamed to 'Paid'
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