Added autoplot option

Automatically plot an image when the z-axis value is changed.
Added a separator to the PlotDialog
Added an Import TIFF Stack option

Imports every image in a TIFF directory (at least initially). Needed to
move the standard import file and button boxes into separate functions
so that they don't interfere with the import directory box and are
called only when needed.
Improved parsing of NXfield dtype argument

Now it is possible to use np.int16, np.float32, etc. as valid arguments.