Docking backwards as plugin param

[cgasconb]

Feature request

The dock_backwards parameter as a dock_plugin parameter

Feature description

The dock_backwards param is a general param of the node, so it is fixed once the node starts. This way, the dock_plugins and dock instances are always forward or backward. In the case of needing a dock_plugin as forward and another one as backward, it is necessary to start 2 nodes changing this parameter. As this param is called each time the velocity is calculated, it would be great if this param is part of the dock_plugin params, so we can have 2 plugins for forward and backward in one node.