Add a Notebook class which we then use to pack in separate Hierarchy
and Properties tabs.
Split out the inspector into its own class to avoid bloat in the
main class.
5 seconds is should help ensure we lose work less often on
Alt-f2 restart.
Avoid saving empty lines to history, and filter them out if
we find them.
Minor fixes for the still-not-enabled inspector.
Add a dropdown pane triggered by Alt-F2, "lg" which supports interactive
JavaScript evaluation, saving/restoring a history file, as well as
an inspector element to pick by using the mouse.
