decision: add a relative date to year concept

The plan file accepts relative calendar date like this "+YYYY-MM-DD"
where +YYYY stands for the number of the year. For exampple "+0-3-10"
stands for the third of february of the year n of the load date of the
plan. And "+1-11-5" stands of the fifth november of the year n+1. The
29 february is not allowed. Reklative numerical date are still
allowed.  Inside the modeling plugin. The Activity Dialog window allow
this new feature.
decision: fix the rename feature on each item

When renaming an item, the change is propagated as far as possible.
The limitation remaining is that source code hand writed is not parsed
to replace the vocabulary that could have change.
decision: add a prefix concept

When a plan is loaded and a prefix is associated to, each activity
name will be prefixed with the prefix of the plan.