Fix top surface gap fill filter

[lin-xianming]

Fix #9246

[hotellonely]

This shouldn't be mixed with the settings made for Sparse Infill. It should have its own settings option and its own states variable.

[Scootworks]

Fix #9246

I appreciate your quick fix.

I believe @hotellonely means that this setting might be confusing because it's listed under the sparse infill section. It would be clearer to place this option in a different section, such as 'Advanced' or a new one.

[lin-xianming]

There are enough settings in the wrong category that I didn't even notice. But I'll leave it up to Bambu devs to decide on UI changes. It's not really clear where it should go, because gap fill is used to both ensure the strength of perimeters in classic mode and also the quality of the top surface when using monotonic lines.