Replaced all calls to "class.isInstance" with the "instanceof" keyword.
Make the RootVisualPanel responsible for managing the modal background.
Bug fix: If you press escape twice, you have to press the close button
on the inventory UI twice to make it go away.
Make the MessageBox class extend the Border element.
Make the inventory UI modal.
Create a list view UI element.
Created a WindowPanel class to base the MessageBox and inventory UI off
of.
Created a GridViewPanel to split the inventory list from item details.