Make plasmoidviewer have non-persistent applet config files.

Non-persistence is on by default, since this is a debugging application
(I've run into a few times already where my applet was loading config
file entries, and they were overriding the less-heavy default settings.
iow: I was getting repeticious undesired behavior.

Enable config persistence by passing --persistent-config

(we could also make the option behave inversely, if that's more
intelligent. seemed like this would be the most desired behavior for a
debugging application).
CCMAIL:plasma-devel@kde.org