Add option to change smooth kernel

**Is your feature request related to a problem? Please describe.**
Using /smooth I can't control steepness of hills generated from raw solid cylinders very well. The generated hills always have very flat tops and almost no transition from flat ground to them.

**Describe the solution you'd like**
Additional parameter to the /smooth and smooth brush, that would take a name of one from couple predefined kernels. Additional ones, that would not be especially for blurring, could be added thru the config files.

**Describe alternatives you've considered**
Giving size for of the blur/smooth kernel would also be fine, but I think that forcing all this kernels to be only for smoothing would be a waste.

**Additional context**
Blue - original terrain
Orange - /smooth X - Current behavior
Black - /smooth X 11x11 - Smooth with different blending kernel