Fixed Numpy import

Not sure how that crept in, but it’s fixed now.
Set focus to imported file in the tree

Similar to the earlier commit concerning opening files
Fixed import bug

In refactoring the dialog classes, I moved the setting of self.accepted
to after the import_data call, which prevented it from being run.
Restricted "Show Link" contextual menu to links

Unless it’s a link, there is nothing to show.