Allow sharing of to-do lists and simultaneous editing

[pyrige]

While the current implementation of to-do lists as user flags is exceedingly simple, it makes it harder to add certain features such as allowing for multiple owner or simultaneous editing.

Using the Entity system should allow us to do exactly that and will let players share their tasks with other individual players or the whole group.

A rework such as this probably would be incompatible with the current design and would warrant a major version jump.

[karpana]

I figured it would be good to know that I too am interested "shared To Do List" that all players and GMs could participate in.