Max height for heightmap

[antzGames]

Added a minMax field to HeightMapTab.kt

If left blank, or set to zero (which would generate a flat terrain) then a default value positive 100 is set.
If user sets the minMax value to negative then the heightmap is inverted.

I think the Terraformer class does not allow negative minHeight so that is why I only implemented one min/max field.

See #174 (comment) for more info and screenshots.

[Dgzt]

Looks good. I think a tooltip with some information would be useful like in settings. What do you think?

Edit: The text and input field are one class, maybe not easy implement into the tooltip. If not easy then just keep this comment.

[JamesTKhan]

I agree a tooltip would be preferred if feasible, to explain the behavior like when the value is negative.

[antzGames]

Ok I added more detailed information:

So I removed negative values. If the user enters a negative value, then I make that value positive. Empty or zero values defaults to 100.

Let me know if you have any more suggestions.

[JamesTKhan]

Looks good on my end, thank you!