GitLab

Perhaps I should revert some part...

Prover ids are only used for the command line option "-P".
The user can choose what he wants (they must be unique)
The prover name and version should not be modified. If someone want to
test different command line options for a prover he can use the
"alternative" field.

If someone want to replay an external proof but he doesn't have the
corresponding prover (same name,version,alternative), why3ide ask for
a replacement among the known provers. The choice can be saved.

 - goal name are correctly printed when no explanation are provided
 - explanation becomes an abstract type

 - create a session if the directory doesn't exists
 - If the directory exists but the xml doesn't behave as if the xml is empty.