Plugin system

Improve plugin system. Scanning for plugins still has to be implemented for Windows, either by using a cross-platform implementation of dirent or by directly using WIN32 API to list files
is scan the action that (re)scans for plugins and fills the ```plugins``` member? if so, i suggest to move the folders where to scan to an extra function, sth. like ```.setPaths(...)``` or ```.setScanPaths(...)``` - probably with vector of std::strings as argument.