Implement sharing of todo lists
It shall be possible for a user to share a todo list with another user of the same ownCloud instance. The user shall be able to set a sharing mode:
- Another user is able to use the todo list but cannot reshare it.
- Another user is able to use the todo list and in addition can share it with other users.
The existing APIs shall remain unchanged and just transparently return also objects shared with the currently logged in user (i.e. using todolist/1.0/delta_get shall return todo lists created by the user but also todo lists shared with the current user). The behaviour of the existing APIs for deletion shall also be enhanced:
- If the owner of a todo list deletes it, it shall be deleted for all users.
- If a used "deletes" a list shared with him, the list shall be "unshared" with that user.