QLineEdit: Extract Method shouldShowPlaceholderText()

The condition under which the placeholder text should be shown changed recently,
so refactor the code to centralize the condition.

It turns out that setPlaceholderText() and paintEvent() disagree on the condition
already.

Change-Id: Id193a9a138042ab0690082dcf634cf7487a5276e
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>