Made a method to open Manage Inventory windows on button click
Unknown change

So apparently I made a change a few days ago that didn't get commited
o.O this is awkward.
Made changes to GUI

see title
Merge remote-tracking branch 'origin/Interface-Mockup' into sprint-1
Deleted Copy
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