Transitional: Do not merge from
Just a transitional commit to save progress while I work.

Currently I've converted the UI for the inventory and popup to respond
to keyboard commands.  I haven't finished linking the popup menu with
the actual commands it needs to execute.  Also, in this commit is the
HealthItem class which extends from InventoryItem.

The Slots for both the inventory grid and popup menu are now separate
custom widget classes.  I had to do this to override the OnKeyDown
method within each.  They are named The_Shell_InventorySlotWidget and
The_Shell_PopupButtonWidget respectively.  I will likely be doing the
same for the Info Box widget as well, though that depends
on what I will have to have it do.  Separating these out has the
advantage of allowing for deeper tracking of the items in the slots.