Add new options to game setup

[historic_bruno]

A few more options that are useful for game setup:

• Speed - set game speed / simulation rate (dropdown: default, slow, medium, fast)
• Resources - set the starting resources for all players (dropdown: default, low, medium, high)
• Population - set max population for all players (dropdown: default, 50, 100, 150, 200, 300, ...)
• Difficulty - set AI difficulty (dropdown: easy, medium, hard, hardest)
• Starting phase / techs
• RMS advanced options
  • Possibly let random maps control what settings are available to the user during setup (change available map sizes or number of players)
  • Add advanced options (e.g. forest cover, water/land balance, resource placement)
  • These could be specified in the corresponding JSON file.
• AI advanced options
  • Mode (aggressive, defensive, etc.)
  • Could be defined in the same manner as RMS advanced options

[Philip Taylor]

Maybe AI difficulty/mode settings should be per AI player, rather than a single global setting that applies to all AIs. (Or maybe that'd complexify the UI too much in the common cases.)