Don't consider frame gap above/below when laying out pages

[miiizen]

Resolves: #13766

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[oktophonie]

I'm not sure I like this solution. One should be able to change the value even if, in the current context, it has no visual effect. Maybe a relayout causes a frame to end up moving to the end of the previous page, or to the top of the next, and you want to clear the (now redundant) value, but you can't because it's been disabled, for example. Or, for a frame at the end of a page, you might want set the gap below 'pre-emptively' for a situation where things have reflowed and it's no longer at the bottom.

I think it would be better to fix the root problem, and prevent 'gap below' from being taken into account when the frame is at the bottom of the page. But the user can still change it, and if the frame is later moved, the value they have set will be relevant. (And same for 'gap above' when a frame is at the top of a page.)