Signed-off-by: Trần Ngọc Quân <vnwildman@gmail.com>

Todoist is a provider that takes special effort on maintaining
the task positions, and in fact it uses the position to determine
the parent/subtask relationship.

So, to make things even funnier, let's add the tasks with the
correct position set.

This makes the Todoist provider look exactly the same as
the web UI. More providers will follow up soon.

This will be used later to set custom sorting.

This makes the ECalComponent of the task the latest available
from EDS.

See #149

This also makes the task list view code more resilient
to backend changes. This should fix the task clones that
are added when a task is modified by evolution.

We can avoid a O(n²) by using the internal
hash table of GtdTaskList

See https://developer.gnome.org/hig/stable/typography.html

It means Bad Gateway, which means "Something happened between
your ISP and Todoist servers, but it's not your fault", which
can be interpreted as "try again and it might just work!".

It is up to the provider to apply or not the changes. Now,
the widgets themselves won't modify the tasklists, so the
providers must add/update/remove the tasks from tasklists
manually.

This is the last big problem with the current design, and this is
finally being tackled here. The correct way to report backend problems
to the user now is by using GtdManager:show_error_message(),
optionally passing an action to be performed.