Refactor random gamesetup dropdown item

[elexis]

In various dropdowns of gamesetup.js the first element is a virtual "Random" option that if chosen, picks a random other item of the dropdown upon game launch.

These are currently:

• player civs
• random map
• biomes

We can imagine many more use cases, even one for every dropdown (for instance victory condition, map size).

This behavior could be refactored, so that a simple "random": true property in g_Dropdowns would suffice to add the colored "Random" item and remove a lot of hardcoding.

[elexis]

I still think that it's more intuitive to have all civs with the same probability, but last time this was discussed there was opposition, so won't fix for this dropdown item.

The caption may differ depending on the translation, "Random" in the biome dropdown might be a different "Random" than in the map selection (might be an negligible difference).

The tooltips also may differ, but could be rephrased.

The duplicated logic is 1-2 lines per setting in the current gamesetup and would remain that few in a cleaner version.

So currently it seems better to let each setting implement it's own "Random" choice.