The two new config keys define a minimum vertical shell thickness.
The top shell thickness is calculated as a maximum of sum over
top_solid_layers * layer heights and top_solid_min_thickness,
the bottom shell thickness is calculated as a maximum of sum over
bottom_solid_layers * layer heights and bottom_solid_min_thickness.
The results of the formula above are shown at the Print parameter page
below the two new values to hint the user about the interaction
of the old versus new config values.
top_solid_min_thickness has no meaning if top_solid_layers is zero,
bottom_solid_min_thickness has no meaning if bottom_solid_layers is zero.
