pygimp: Add no run-mode parameter to register function && Fix OpenRaster load thumbnail procedure

This merge request refactors plugin registration function in ginpfu.py to add an extra option to not define 'run-mode' parameter by default. Motivation for this was function file_open_thumbnail which doesn't have parameter run mode and it failed to call the underlying procedure for loading thumbnail of specific file if thumbnail plugin was made with gimpfu (because of an extra run-mode parameter).

This MR also fixes thumbnail loading procedure for OpenRaster file format.