Changed double-clicks to generate plots

I've made the tree items non-editable because I want renames to be a
more deliberate action.
Tidied up behavior of matplotlib windows

Now, plot windows should exit cleanly and their menu items should be
removed. Also updated init_toolbar to reflect changes in the matplotlib
base classes.
Make references to NXfield attributes return scalars where appropriate

Before, they returned array scalars, but this upset the sorting of axes
specified with the now-deprecated 'axis' attributes. Sorting of keys
based on NXattr objects containing integers failed so it was necessary
to return a scalar integer instead.