Layered meshes with minimum thickness layers

Based on requests by Agnes and Marc I added a parameter to the layered mesh generation that allows to specify a minimum thickness for all layers (i.e. if interpolated raster data suggests that the layer is too thin at certain locations it is assumed to be non-existent).

Most of the changes in this PR are just formatting things where I changed spaces to tabs to be compliant with the overall code layout. Sorry about that.
I do not see the point where min_thickness_validator is used. How is the memory management? Do you need to delete this local pointer variable or is it some Qt magic that frees the mem?