Client : implemented new "ui_drag" command for drawing a dragged object
along with the cursor.
Client :
1. use the "grab" hand cursor while dragging
2. added UI_ClearDrag() function
Game : added 'item_kind' entity field and ITEM_XXX enums.
Game : more work on inventory system (server-side), adding INV_GiveItem(),
INV_CheckHasItem(), INV_Recount() functions.  Also added 'inv_count' field.
Game : more work on inventory, have a linked list of entities for each
player entity (same as what we do on client), each with 'invent_id'.
Added code to create this list, find a free slot, find slot by ID, etc...
Inventory : send 'ui_invent' updates when giving or removing an item,
and implemented INV_SendAllToClient().
Game : fixed to use stuffcmd() to send "ui_highlight" command to client.