dump config on prefix argument to set_config
mark the new window for redisplay when we split
log from the GapBuffer properly
start at the beginning of a buffer (except for prompts)
insert-file

(for rapidly getting test content into an editor)
Keep track of the frame by chunk-cursor + lines of offset

So we can do pagedown (and eventually pageup) more properly with window
types that return chunks larger than a screen line.  Also give the windows
less reach for abstraction violations.