Possibility to choose which templates to use

As Migrating from Jahia 4 to Jahia 5 requires to modify the templates, it would be useful to be able to choose which templates to use. Currently, the jar included in the export archive is used. So we have to extract this jar, modify it, and put it back in the archive. This last operation is not very user friendly, and most of all, it can lead to some problems of archive corruption.

So it would be easier if we could just put it in /shared_templates/ , and then, when importing, be able to choose between using the jar included in archive (default choice) or to select an other one to use.
In addition, it would let us provide a corporate jar used to migrate Jahia 4 sites using corporate templates. In this case, maybe we should add a parameter in templates.xml to indicate that the set of templates can not be used for site creation, so that the migration jar would be only used for migration.