lupdate has issues with resolving non-trivial uses of "using namespace"
(QTBUG-64007)
Move "using namespace" directive to a place where it is better handled
by lupdate.
Also use TextEditorWidget instead of *Private as the context.

Change-Id: I7d3bb8e1bc493196e47085827be31f97e0dce7b8
Reviewed-by: David Schulz <david.schulz@qt.io>