Worked on property change events in this commit.
1. Made Quantity and Rate textboxes validate by a Validate event to change values to 0 incase the field is empty.
2. Created class ProductChangedEventArgs with properties mirroring the values of Rate and Quantity for use by event handlers in main form.
3. Created event and delegate objects as well as OnChanged method which calls all callbacks subscribed to the event.
4. Subscribed a method to the event in Main form, yet to do something concrete with it.
Merge branch 'BasicDesign' of github.com:VaticanCameos/MaharajaDataEntry into BasicDesign