Changed RelativeLayouts to LinearLayouts to fix formatting
Made ListView correctly show items, added listeners for
selecting/deselecting and deleting. Implemented Item, ItemAdapter, and
FileManager.
Made checkboxes correctly sync with Item's checked property. Added
deleting via long click on an item.
Finished ItemManager implementation, added options menu, implemented
swapping between archived and unarchived items, refactored anonymous
listeners to be named classes existing outside the main activity. Fixed
FileManager so it properly syncs app data with the save file every time
something changes.