apl_gradebook: Add entry list ptr

Will point to an array of entries. These will probably be sorted
by time created, just to keep it simple. To display different sortings,
we'll likely just create index arrays which represent different
sortings (ascending/descending score, date created, etc.)
apl_gradebook: Add apl_closebook skeleton func

Free gradebook memory & resources.
Merge branch 'entry' into gradebook