#52 popup function

[aaron brick]

we need a generalized popup function for reporting
warnings and such. these windows approximate dialog
boxes, because they need to feature option choices
("continue" or "abort", in the case of a warning about
input style).

i can't think of any other cases right now. we can
refactor the button meanings later if need be.

[aaron brick]

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ok, more cases: success reporting (a single button to close
the window and browser goes home); fatal error reporting (a
single button to close the window and browser stays put). we
can finally replace SuccessPage and ErrorPage!

all the buttons close the popup. this suggests a few types
of buttons: do nothing, move on (to a specified page), go home.