Pressing the 'Add Item' button now clears the text in the 'edit_string' EditText view.

Implementing 'updateItem' within 'ItemDataSource'.

Implementing 'markItemDeleted' within 'MainActivity' that removes an Item from the list,
and also sets the 'isDeleted' column to true.

The CAB now deletes multiple items using the 'markItemDeleted' method.

The 'strikethrough' onClickListener now updates the respective Item toggling
the 'isMarked' property.

'getView' within 'SelectionAdapter' now properly sets strikethrough according to if the
respective item 'isMarked'.

Adding 'layout_weight' properties to all views in 'listview_row_item.xml' to increase
redability.

Cleaning up unnecessary package imports, and old, unneeded commented code.