Support sub requirements in `Requirements table view`

[jerome-obeo]

Currently, the Requirements table view only displays the root level requirements found in the selected namespace element.

The goal of this enhancement is to display all requirements shown as a hierarchical table (nested requirements can be revealed under the owing requirement).

[AxelRICHARD]

Please also add a new "New Sub-Requirement" Action on an existing RequirementUsage:

The "Import all existing Requirements" tool should now also import Sub-Requirements:

Also don't forget that RequirementUsage visible in the table are exposed in the associated ViewUsage.