Better support for off-resonance R1rho data in the dispersion GUI.

A new row of buttons has been added to the dispersion GUI, just above the Peak list GUI element.
The first button is for the spin.isotope user function and replaces the old GUI element.  Two new
buttons for loading R1 data and chemical shifts have also been added, as required for off-resonance
R1rho data.
Changed the chemical shift icon to that of the chemical shift in ppm units - the delta symbol.