It doesn't need the editor parameter anymore, so the two-phase
setup can go. Note that both paths through ctor() finally hit
*Widget::createEditor() (one directly, one indirectly through
the first BaseTextEditorWidget::editor() call), so this doesn't
change what's set up, only when.

Change-Id: If92a7e2a60ea3df417ecdeabc6f8e9dc3a23e4f8
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>