[Re]name sidechain port directly when a plugin is added.
Usually plugins are constructed without an Route (owner is unset).
PluginInsert c'tor may already create a side-chain port, at the
time of construction the sidechain port will be created using
the port-name "toBeRenamed".
Previously the plugin had to be added to a route using "add_processor",
in order to set a unique name, before a new plugin with sidechain
could be constructed.
ProcessorBox::use_plugins() did that in the correct sequence,
however there may have been cases where this didn't work, and
Route::add_processors() was called directly..
46a6f47362