DistUpgrade: do not make UI dialog calls while EUID != 0

The screen lock prevention code has to temporarily set the EUID to the
non-root user UID so that it can make a DBus call on the user's session
bus. While the EUID is temporarily non-zero, a UI dialog may be
displayed to report the success of inhibiting the screen lock. For most
cases this is OK, but it causes an exception for the DistUpgradeViewKDE
frontend. This is because the view.information() call needs to read
dialog_error.ui from the tmpdir, but the non-root user does not have
permission to read this file.

To fix this, in _inhibitIdle(), move the information dialog call to
after the finally block which re-sets the EUID.