Add option to change smooth kernel

[Lukasz825700516]

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