Add a `Delete` button to the GUI NavBar, and cleanup deletion callbacks.

Add a `Delete` button to the GUI NavBar, and cleanup deletion callbacks.

The most confusing part of this change is the adjustment to
`Default.txt`.  The NavBar was not expanding to display the new button
(which was avialable via a flyout), so I had to manually adjust the
default perspective.  Because we load the default perspective after
setting up the GUI, any changes in the code (e.g. setting a reasonable
width) get clobbered later by `Default.txt`.  Anyhow, the change I
made to `Default.txt` was to change the toolbar's `bestw` from 100 to 300.