Updated shell dictionary when a tree node is renamed

A nexpy.api.nexus.tree method allows NeXus nodes to be renamed. The new
name is propagated to the tree view but the IPython shell dictionary
was not updated for root nodes so you couldn't reference the renamed
variable in the shell.
Added a new global variable _mainwindow

This is for the new import dialogs.
Dynamic import dialogs

The changes allow import dialog modules to be added dynamically to the
Import menu and provides an example for those customizing data imports
from non-NeXus formats.