Set margin_top on the revealer bars in addition to margin_bottom

[ibuclaw]

1. Sets equal margins on both top and bottom, which (to me) has a better aesthetic look to it.
2. I noticed that there was a typo in the code which meant the bulk update ("multi") revealer never got a margin set.

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Before this PR:

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After this PR:

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[ibuclaw]

After a little toying around, I've convinced myself that a slightly smaller margin for the yes/no buttons looks better.

Updated "After" screenshot:

[jorsn]

Thanks!