todo-txt: Improve and simplify Parser
This commit perfroms following:
- Ensures todo.txt use utf8 format so that languages other than English can also be used.
- Simplify token id detection. For eg. task title can now contain any character and not just alphanumeric. This is done by using last_read token id and a flag title_read to denote if title has been read.
- Remove restriction on some tokens coming at predefined location. This provides us flexibility to use.
Issue link: https://gitlab.gnome.org/GNOME/gnome-todo/issues/58