Add an item bar for a quickly access to items.

Some insight about it:

• It must be movable when the user has done a /slt unlock, similar to tracker.
• it must not be anchored or parented to Tracker for avoiding issues related to Secure Frames.
• add an API for the other modules, allowing them to add their items into the item bar.
• add a sorting feature. By example, the Quest module can use that for sorting the item quest in function of the quest distance.