core: keep using default_params from proxy listbox ids

With bf57228a ([erp5_core] RelationField listbox must also fetch
default_params value from the relationfield, 2018-05-09), we introduced a
regression that listboxs from relation fields "proxy listbox ids" default params
where no longer used. As a result, if we have a listbox which define some
catalog search with default parameters, like for example the "Suppliers" or
"Clients" listboxs from erp5_trade are doing, these parameters were not
taken into account. In the case of trade listbox, this was showing all nodes.

This changes Base_getRelatedObjectParameter to get catalog search from the
proxy listbox if there is one.