Expanded the Relax_disp.test_hansen_trunc_data GUI test to check the spectrum list GUI element.

The popup menu is now tested with the Fake_right_click() trick.  And the actions of a number of the
menu items, the action*() methods, are tested to see if the user functions are correctly called.
Modified many of the spectrum list GUI element action_*() methods for the GUI tests.

These now take the 'item' keyword argument which overrides the ListCtrl.GetFirstSelected() call.
This ListCtrl call cannot be reliably simulated on all operating systems, so the item keyword
argument can be used to explicitly select list items.
Fix for setting the relaxation time in the spectrum list GUI element for the dispersion analysis.

The popup menu item was calling the relax_fit.relax_time user function and not relax_disp.relax_time.
Fix for the action_relax_disp_cpmg_frq() method of the spectrum list GUI element.

The relax_disp.cpmg_frq user function was being incorrectly called.  This was identified via the
Relax_disp.test_hansen_trunc_data system test.
Modified the Relax_disp.test_tp02_data_to_tp02 GUI test to check the spectrum list GUI element.

The popup menu is now tested in the same way as in the Relax_disp.test_hansen_trunc_data GUI test.
Modified the spectrum list GUI element action_relax_disp_spin_lock_field() method for the GUI tests.

This now accepts the optional 'item' keyword argument like the other atcion_*() methods.
Bug fix for the spectrum list GUI element popup menu relax_disp.spin_lock_field entry.

This was calling the relax_disp.spin_lock_field user function incorrectly.  The bug was identified
by the Relax_disp.test_tp02_data_to_tp02 system test.