As a user, I can add Tests to the Test Plan board, even if they are not part of the Test Plan

As a user, I can add Tests to the Test Plan board, even if they are not part of the Test Plan.

An option in the Add Tests dialog (checked by default) should restrict adding Tests to a folder, even if they're not in the Test Plan; it should be possible to uncheck it and thus remove this restriction.

The "Add Tests" folder action should be available for the "All" and "Orphans" meta-folders; in this case, it should implicitly allow to add Tests that are not yet on the Test Plan. Thus, it will be used to add Tests to the Test Plan.