#723: fix section add bugs

This fixes #723 - don't allow users to add blank sections in the dialog or via the internal add section routine. This also addresses adding sections that already exist.

There is a minor tweak to the add dialog section auto-complete to stop it from suggesting (invalid) section starting text such as `env:` when the add dialog is launched from a right click menu at `env`.

@arjclark, please review.