Add: allow setting the highest mountain for heightmaps

It will add some slack to the map height limit if that was set
to auto.

Change: remove "maximum map height" from the New Game GUI

This setting influence the max heightlevel, and not as the name
suggests: the height of the generated map.

How ever you slice it, it is a very weird place to add this
setting, and it is better off being only in the settings menu.

Commits following this commit also make it more useful, so users
no longer have to care about it.

Feature: setting to indicate desert coverage for tropic climate

This is an indication value; the game tries to get as close as it
can, but due to the complex tropic rules, that is unlikely to be
exact.

In the end, it picks a height-level to base the desert/tropic
line on. This is strictly seen not needed, as we can convert any
tile to either. But it is the simplest way to get started with
this without redoing all related functions.

Feature: setting to indicate snow coverage for arctic climate (replaces snow line height)

Setting the snow coverage (in % of the map) makes a lot more sense
to the human, while still allowing the niche player to set (by
finding the correct %) a snow line height they like. This makes for
easier defaults, as it decoupled terrain height from amount of snow.

Maps can never be 100% snow, as we do not have sprites for coastal
tiles.

Internally, this calculates the best snow line height to approach
this coverage as close as possible.