Also make a pretty printer for a single todo.

Instead of numerous booleans that steer the pretty printer, let the
caller pass on the desired transformation function.

This will mark the todo list dirty.

add_dependency would store them as strings. On the next invocation
of add_dependency, find_new_id wouldn't find the previously stored
value since it looks for integers. Therefore it returns the same
ID, resulting in duplicate edge IDs.

There's a difference between 'text' and 'source text', where the former
is a more user-friendly representation of a todo, while the source text
is the raw text from the todo.txt file.

Reflect this difference in the method name to avoid confusion with the
'text' method.

When passing False for case sensitivity, it would fall into the logic
to automatically determine whether case sensitivity is desired
based on the appearance of capitals.

When using the add_todo method, the todo number is not set.
It needs some refactoring to move the number property to the
TodoList, instead of being a property of Todo.