todoist: save local changes to todoist

This patch implements:
    1) gtd_provider_todoist_remove_task_list
    2) gtd_provider_todoist_update_task_list
    3) gtd_provider_todoist_remove_task
    4) gtd_provider_todoist_update_task

With this, todoist tasks/lists can be modified
from To Do and the changes gets saved on Todoist.

https://bugzilla.gnome.org/show_bug.cgi?id=772278