Re-order mapgens in mainmenu and 'all settings' mapgen selection

[paramat]

Text for each mapgen is still a good idea and will probably be done, but a logical display order in the mapgen selection dropdowns is also useful.
Somewhat attends to #8644

// Order used here defines the order of appearence in mainmenu.
// v6 always last to discourage selection.
// Special mapgens flat, fractal, singlenode, next to last. Of these, singlenode
// last to discourage selection.
// Of the remaining, v5 last due to age, v7 first due to being the default.

New order:

	{"v7",         true},
	{"valleys",    true},
	{"carpathian", true},
	{"v5",         true},
	{"flat",       true},
	{"fractal",    true},
	{"singlenode", true},
	{"v6",         true},

Promotes the newer mapgens Valleys and Carpathian more. The first 3 in the list are the 3 most impressive mapgens.

[paramat]

Now tested, does not cause problems for existing worlds, mapgen selection works fine.
Ready for merge.

[SmallJoker]

But people might argue now that these are totally out of order in terms of alphabetic sorting.

[v-rob]

I think that v5, v6, and v7 should still be renamed to something else, as do many other people, but I know that is a subject of great contentiousness and I'm not going to press the issue.

This is a good idea, especially since dropdowns shows only 5 options before you have to scroll down, so the five most important are the first ones people see, and if they never scroll down, they won't even notice the more specialized/v6 mapgens.

[srifqi]

What about this file?

minetest/builtin/settingtypes.txt

Lines 1328 to 1332 in ecd20de

	# Name of map generator to be used when creating a new world.
	# Creating a world in the main menu will override this.
	# Current mapgens in a highly unstable state:
	# - The optional floatlands of v7 (disabled by default).
	mg_name (Mapgen name) enum v7 v5,v6,v7,valleys,carpathian,fractal,flat,singlenode

[paramat]

Heh, it was already not alphabetical order, and there's no need for that =)

srifqi, thanks, 'all settings' displays a dropdown menu too, will change that.
EDIT: Done.