Reworked Pickup System design

Started to redesign the entire process from item pickup to placing in
inventory.
Implemented Slot Tracking

Slots are now tracked as being empty/full.
Renamed a couple functions in InventoryWidget

Renamed RebuildGridLayout and PopulateGrid functions to more
reader-friendly names.
Temporary Fix for Issue #3

Introduced hack to resolve issue #3 (InventoryGrid not displaying item
icons till after ToggleVisibility function is called once) till a more
appropriate answer is found.
Item Icons!

Items can now be assigned an icon, which is what represents an item win
an inventory slot.
Update README.md
Update README.md
Update README.md
Refactored how items are stored

InventoryWidget no longer contains a list of RItemInventorySlots.
Instead it is retrieves the player's inventory during construction (if
toggled visible) and updates itself.
Merge branch 'development'