let template overwrite configuration

[cmb69]

That's really important, because certain templates might need special configuration. OTOH, we want still allow template designers and webmasters to use the supposedly simpler way of setting config options via the back-end, at least during the design phase.

See also the related discussion in the forum.

[cmb69]

Of course, this can already be done by simply overwriting the respective global variables in the template, but we want to insulate from eventual changes in the general plugin configuration handling, and also from eventual changes in the concrete plugin configuration of Onepage_XH. Therefore we need a small API, which has to be well documented.