Set focus to opened file in the tree

It is likely that, when a new NeXus file is opened in the tree, the
user will want to inspect it. Selecting the file and setting focus to
the tree view saves a mouse click.
Add __delattr__

This allows NeXus objects to be deleted using attribute references
rather than the dictionary. This commit also removes the _saved
attribute that is now redundant with unlocked HDF5 files, where the
file update is immediate.
Refactored tree view

Before this, each NeXus node had an _item attribute linking it to the
tree view item. However, this led to crashes when the tree view item
had been deleted. The new implementation synchronizes the tree view
with the ‘tree’ node whenever a node is changed. This leads to much
simpler code and, so far, more reliable behavior.