request #7491: Table columns are not saved during XML tracker import

Beware, here be dragons.

During XML import, columns are created in session. However it lacks some
information like the field id, which is pretty normal since we do not
have an id during the import. This means that we cannot use getColumns()
method because it relies on the presence of the field id for each column
to be build.

Hacking getColumns() to return the internal collection of columns (with
the reference to the field object) is not an option since it breaks
things.

The trick here is to delegate to the temporary renderer object to inject
the columns in the real one.

(╯°□°）╯︵ ┻━suoᴉssǝs━┻

Change-Id: I9e2b9713ee91093cc42ee0c4919bc405a69407b8
Merge commit 'refs/changes/64/3064/1' of ssh://gerrit.tuleap.net:29418/tuleap into tuleap-stable-master

* ssh://gerrit.tuleap.net:29418/tuleap:
  request #7491: Table columns are not saved during XML tracker import

Change-Id: Ic7c2c9b4a156eb9a3d02d8aee7440a0e565a76de
Tuleap 7.5.99.20

Change-Id: I921bb2325d254b8a5f744b6f55dccffbea678083