Add support for custom combos in PrintProviderField

Hello

as for now, in plugin PrintProviderField, only the standard layout/dpi/format combos are taken into account. Any other custom combo is ignored:
https://github.com/geoext/geoext/blob/master/lib/GeoExt/plugins/PrintProviderField.js#L172-L180
whereas it is possible to add any other field (textfield for instance) thanks to:
https://github.com/geoext/geoext/blob/master/lib/GeoExt/plugins/PrintProviderField.js#L183

This PR adds the support of custom combos.
what is the reason for using valueField as well? Other parts of the code only deal with field.name