Some users wondered why NEdit uses Ctrl+B to toggle
insert/overwrite, when it seems that every other app in
the universe uses the Insert key.
Adding further weight to the argument: Motif XmText and
XmTextField widgets both use Insert to toggle this
This patch does two things:
1) Adds [Insert] as an accelerator for toggle
2) Frees up Ctrl+B for something else.
Both items are orthogonal, so we can pick and
choose what to do. However, I can't really think of
any reason to keep Ctrl+B except for history. Moving
to a more standardized accelerator feels right.