Unify gamesetup option handling, fixes #3994, refs #3049.

Removes a lot of duplication and ugly GUI handling code with very similar, yet different code paths.
Move the setting specific logic to the functions of that setting and keep the universal logic in global atomic functions.
Make gamesetup.xml agnostic of all gamesetup settings.

Adding a new gamesetup option now only requires adding one hunk with the titles and values and one line in g_OptionOrderGUI.
Opens up the possibility to iterate over all settings, refs #3806, #3883.

Allow starting singleplayer games in observermode with only AIs assigned, fixes #4078.
Autocomplete translations of all setting titles and selected title values like playernames and victory conditions.

Transfer the AI difficulty when swapping with a player.
Move logic from onTick to the GUI handling functions.
Change some global consts to var, so that contributors become invited to change them from a different place.
Add missing startGameButton tooltip translation.

Differential Revision: https://code.wildfiregames.com/D322
Reviewed By: wraitii
Some strings reviewed By: leper