tasks #7403 Export / import XML

This task is part of story #7161 Create a date-time field

Display time attribute is now exported in tracker XML structure and is
taken in account during import process

Change-Id: I2fc1da497b20eee86b75f634a758a84e6975c207
Merge commit 'refs/changes/61/2961/1' of ssh://gerrit.tuleap.net:29418/tuleap into HEAD

* ssh://gerrit.tuleap.net:29418/tuleap:
  tasks #7403 Export / import XML

Change-Id: I68b3b88c65e2e216f4398219008305557cdbfa9a
This is Tuleap 7.4.99.15

Change-Id: I236b9e788f5a9a2edfb80f4af9ac797fcd77881b