Add items collection

[tomgreenfield]

Proposal: use a true Backbone collection for items in a component model.

Benefits include built-in change events and helper functions, and it also works to align with Adapt's existing parent > collection of children architecture.

Building on the excellent foundation from @lc-thomasberger, the functions have been whittled down to the below:

Component level itemsComponentModel.js

• init(): custom initialise function to set up items and event listeners
• setUpItems(): create a collection of ItemModels
• getItem(index): get the item at specified index
• getVisitedItems(): return all items where _isVisited: true
• getActiveItems(): return all items where _isActive: true
• getActiveItem(): return first item where _isActive: true
• areAllItemsCompleted(): does what it says on the tin
• checkCompletionStatus(): set component to complete if all items are completed (easily overriden)
• reset(): resets all items and then component
• resetActiveItems: set all items to _isActive: false

Item level itemModel.js

• reset(): shortcut to set _isActive: false and _isVisited: false
• toggleActive(isActive): shortcut to this.set('_isActive', ...);
• toggleVisited(isVisited): shortcut to this.set('_isVisited', ...);