* fixed lint errors in button.go
* fixed lint errors in checkbox.go
* Removed d2ui singleton, fixed nearly all lint errors
- Changed `UI` struct to `UIManager`, removed singleton
- UI element provider functions are now methods of the UI Manager
- Screens now use the UI manager to create UI elements
- game panels in d2player now use the UI Manager to create UI elements
- Only the UI manager knows about "widgets"; calls to `d2ui.AddWidget` in Screen instances have been removed
* changed ui element provider methods from `Create` to `New`
* adding simple inventory item descriptions
* adding method to identify items
* offset description so it doesn't overlap with item in inventory grid
* needed to offset inv panel by 1px
* adding resource entry for inventory.txt
* adding loader for inventory.txt
* adding call to inventory.txt loader in d2app
* d2game now uses the inventory.txt records for making the inventory panel
* Add InventoryItemMisc struct
* Add GetMiscItemByCode function
* Implement InventoryItem interface for InventoryItemMisc
* Add rings and amulets to the example items loading.
* Fix Y of LeftHand and RightHand equipment slots.
* Add "Ripolak" name to CONTRIBUTORS
* Remove double item
* Add basic EquipmentSlot struct
* Add Load function to EquipmentSlot
* Move EquipmentSlot struct to inventory_grid.go
* Add equipmentSlots arg to ItemGrid struct
* Add basic rendering of equipment slots.
* Change rendering of equipment slots to simply use their static x and y
* Add ChangeEquippedSlot function
* Add initialization to all equipped slots types.
* Fix Y locations of equipment slots
* Move default equipment slots to a genEquipmentSlotsMap function.
* Change Item to item
* Fix coordinates
* Change usage of string to EquippedSlotType when dealing with different slots in rendering.
* Fix import error
* Remove neck example
* Add loading sprites of equipped items.
* Clean code in Inventory rendering
* Clean code in Inventory rendering
* Clean code in Inventory rendering
* Change default items that get rendered.
* Change default items that get rendered.
* Add width and height to EquipementSlot struct
* Fill in width and height to current equipment slots.
* Fix Y setting of equipment slots
* Rename variables for clean code.
* Change handling nil itemSprite to condition instead of loop return.
* Split Render function to 2 functions.
* Add TODO
* Change comment to start with capital I
