This allow for example to sort the wall list by context, then each context by
project and each project by priority.
The sort is very flexible, the only constraint is that the sort by priority
(if we decide to do it) is the last sort.
This commit add the first interesting difference from freitass original
version. It provides a two level sort function which allow the user to sort
the todo by project (or context) then by priority.
TODO: Maybe think about three level sort (project, context, priority) in any
order.
