Added MainWindowViewModel
Added IProductService
Added TransactionItem
Added ObservableObject
Revised ProductService Implementation
Added MainWindowViewModel
Added IProductService
Added TransactionItem
Added ObservableObject
Revised ProductService Implementation
Removed empty reference to IProductService
Merge branch 'transaction-viewmodel' of https://github.com/CIT275DevGroup/openTill into transaction-viewmodel
Merge pull request #13 from CIT275DevGroup/transaction-viewmodel

Add GUI Backend for Transactions