Layout grid: improve resize validation experience

[johngodley]

Sometimes you are unable to change an offset/span value because the grid validation prevents that particular combination. However, if you 'skip over' a few numbers then validation is possible.

We should try and improve this situation.

For example, this video shows that it's not possible to reduce the offset from 4. This is because 3 is not valid. However, 1 and 2 would be valid. Maybe we can detect this and skip from 4 to 2.

https://cld.wthms.co/Ckgj0z