Allow main menu to set options for new games

[originalfoo]

Currently most mod options are not changeable from main menu, and can only be changed on a per-game basis while in-game.

Would be nice to update the mod options system so that changes made from main menu are used as defaults for new games.

Furthermore, while in-game, a new button on maintenance tab to "Save as defaults" (save current game settings as global defaults).

A good example of this functionality is the Real Time mod by @Dymanoid (specifically, if memory serves, the Sky Tools git module)

[jpickwell]

This would be a great addition.

[kianzarrin]

dependent tasks:

• #1372: when all fields are in one class, they are easier to search for.
• #1510: when all shared code is on one base class, it is easier to access them using generic code.
• #1508: when everything is encapsulated in one place, it is easier to work with.
• #562 : XML is more flexible and more useful for this task.