Summary:
- let the KXMLGui magic work, and do not disable the Help menu;
- set the translation domain upon creation of the QApplication
  (so the KAboutData is translated too).

The About entry for the current module is not shown yet, and
it will be nice to add it, but this is not a regression compared
to the current status.

Test Plan:
The full standard help menu is shown, with the additional entry
which activates the Kontact start page (which was the only entry
before this patch).
The About box shows the "Translators" field when a translation
is used.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: mlaurent, #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D8097