Add solid_infill_specific_layer expert option

[PetervdPerk]

When printing big objects with low infill rate (< 10%) the gaps between the infill can to big to do a nice transition. This results in 2 problems, either there's a gap in your print, or it curls upwards and the extruder bumps into it which also degrades print quality.

This PR adds the solid_infill_specific_layer expert option where an user can specify a specific layer to force solid infill from perimeter to perimeter which avoid creating holes, whilst minimizing printing time and material usage.

[jschuh]

Don't know if you were aware, but you can already accomplish this by adding a height range modifier that sets the infill to 100%. Height range modifiers also give you a lot more versatility, because you can add multiple of them, cover different ranges, set different infill densities, etc.

[lukasmatena]

@PetervdPerk Thanks, but I am against adding an extra configuration option to hack around one problematic spot. The real solution is to finally solve #569. Hopefully it won't take much longer. Closing.