QT_PLUGIN_PATH (Re: Install kimgio plugins in the Qt image format plugins dir)

[Maksim Orlovich]

> Yes I think this would be a good idea.
> What about the attached patch?

We definitely want this for styles as well... I kept forgetting to do this
for a while... Since about Qt4.0.1 era. krdb used to write the equivalent
into qtrc, but the key is gone now.

-Maks