Moved source list item template definition to item class

Item view templates are no longer inherited from the containing source list view.

This maintains consistency with Ember's built in collection views.
Added `getFlattenedContent` method to multi array controller

Method flattens content across arrays/sub-arrays recursively to return a single array of items.
Improved computation of "active" selection

The "active" selection is typically the intersection of the current selection set and the current content array. That is, an item only appears actively selected when it is both in the selection set and the content.

A new `getActiveContent` method for determining the content array from which active selections must be a member of was added to allow multi array controllers and the selectable mixin to work together.
Reduced selection change event triggering
Made `arrangedContent` computation function length 2