Extend plugin paths

[Nicogene]

This PR aims to extend the plugin search directories, gazebo-fashioned.

TODO:

• Check old functionality(local dir)
• Check new functionality(plugin paths)
• Add documentation

[traversaro]

FYI the sharedlibpp version as modified in blockfactory also contains some of this logic, see robotology/blockfactory#32 .

[Nicogene]

I would start with a quick and dirty solution and see if works, and then the introduction of sharedlippwould be nice, but unfortunately we have to do yet another fork 😞

The best solution would be having a unique sharedlibpp and use it everywhere (https://github.com/icub-tech-iit/tickets/issues/379).

[traversaro]

I would start with a quick and dirty solution and see if works, and then the introduction of sharedlippwould be nice, but unfortunately we have to do yet another fork 😞

Yes, that was just a link that I shared FYI, going forward with this change is definitely the way to go for now.