Project Grouping

[ADD] Now tasks from the same project are grouped together... For the
moment the name of the project it's still displayed in the task's name,
but I'll see later if it will stay here or if I will remove it :)
[FIX] Hide project name from the task

It has no sense in having the project in the name of the task... The
project name will only be displayed if the task is attached to multiple
projects!