Layout Grid: inspector setting validation too strict

[johngodley]

The settings in the block inspector apply grid validation on each key press, making it difficult on smaller width devices to make any direct changes.

For example, if you try and delete the value in a grid on mobile it won't let you as the empty value causes a grid validation error, so it gets reset.

We should only validate once the user has finished making a change, maybe onBlur instead.