Add new Show Link menu item

Also merged "Save" and "Save as…" since read/write files are now
immediately updated
Added NXFile to shell imports
Fixed duplicate attributes in fit results

Caused by using mutable values (i.e., empty dictionaries) as default
keyword arguments
Added customized right-click zooms

Used for projections - the zoom rectangle is used to define projection
limits, but does not rescale the plot. Also, when the z-limits are
locked, steps will be in units of the difference.
Reconfiguration started to allow HDF4 and XML reads

The NXFile class is no longer a direct sub-class of h5py.File so that
it will be able to handle file reads using the NAPI (not enabled yet).