Some of the detected bugs already fixed

1- Number of representatives shown in the scene is now available in the Info Table.
2- The scene is not reset anymore when a new ROI is created.
3- If not file is specified when using an OpenFileDialog (for loading structural image, tractography or saved session) nothing is done. Bug removed.
4- D key for deleting selected representatives has been disabled.
5- Some small things that were missing when loading directly a saved session the first time the system is loaded, e.g. enabling the Loading Tractography option, Saving Session and Saving .trk files, are now available.