Fixes segfault on close. Using the right objective (d_main_gui) instead of d_object...

Fixes segfault on close. Using the right objective (d_main_gui) instead of d_object to delete all of the plotting widgets. As far as Valgrind tells me, this cleans up my memory leaks although libqt and libqwt still have some of their own.

This checkin also disables the Waterfall3D plotting. It wasn't working anyway, but was also causing its own segfault upon deletion. Once it's working and deleting nicely, it will be re-enabled.