more constants for layout

[Habbie]

as noted in #68, we could do with a few constants for the sizes of some things. Those constants could then be device-specific, or maybe even calculated at startup (if we define them the right way, no code will care if they're actually constant). While building #68 I kept accidentally creating scrollbars and we can likely do better. Most of the numbers in there are fudged too.

[cyclops1982]

If we do this, then we need to create like a boxmodel type diagram that indicates which constants are for what. Otherwise this becomes difficult to maintain.