fdo#72947 Startcenter: display more userfriendly file urls in tooltips

Use INetURLObject to get a readable url (it displayes special characters
too). It is a side effect that 'file:///' prefix disappear
which seems a good thing.
Plus some cleanup: remove second INetURLObject.

Change-Id: I4aab5e2546ae70e5a3d85b12e26e021bfa244999
(cherry picked from commit 31dfeee42e9b08ed7031b6c50d0c674122d5245c)
Reviewed-on: https://gerrit.libreoffice.org/7366
Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>