work done:
- refactored ProductScreenController.java to make use of
ControllerGeneral.java
- added code to move to ProductScreen when a product name is clicked,
however, had to move code that generates the cell factory for cells in
the product name column back from ControllerGeneral to individual
controller classes. ideally this should be refactored so that the code
doesn't repeat in each controller.
- added currentProductID field to SmartTrolleyGUI.java as well as get
and set methods. currentProductID is set when a product name is clicked
in the table and used on the ProductScreen to retrieve the product's
relevant data
- added currentListName field to SmartTrolleyGUI.java as well as get and
set methods. currentListName is set when a list is either created or
loaded and is used on all screens to show the currently active list
- added code to AllShoppingListsScreenController and
CreateNewListScreenController to do this

problems faced:
- could not move to product screen with product name column cell factory
code in ControllerGeneral. something to do with not being able to use
non-final variables within an inner class. 
- resize behaviour of search field and search button to do with list
name label

work to be done:
- fix above