Replace QStringLiteral with QLatin1String where appropriate

It's faster to use QLatin1String in places where we don't actually need
a QString object (like for comparison, or when creating a string with
QStringBuilder).

Change-Id: I9b8e28040dc2b79a767e15015b85822d06141bbe
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>