New plugin public methods. Bugfix

• implemented RFM plugin public methods "refreshFolder" and "loadFolder" #275
• bugfix: problem with folder downloading #273
• bigfix: redirect on failed authentication doesn't work #278
• bugfix: fileroot issue with symbolic links #285
• updated PHP connector dependency to v1.2.1
  (https://github.com/servocoder/RichFilemanager-PHP/releases/tag/v1.2.1)