Add TodoList.remove. Project/context indexes point to ids, not indexes

Indexes are as follows (where list_index = index of TodoList.items):

TodoList.indexes.id = { id: list_index }
TodoList.indexes.project = { project: [ id [, id] ] }
TodoList.indexes.context = { context: [ id [, id] ] }

To get an item from a list of project ids, use TodoList.findById(id).

Also added TodoList.deindex(item) to cleanup the index by removing
references. Will prune empty project/context indexes as they become so.