Hub levels

This PR adds a Hub gametype, as well as a "Return to Hub" selection in the pause menu if a hub map exists.

New:

• Added the Hub gametype
• Added HUB type of level
• Added to SOC:
  • DEFAULTHUBMAP for MAINCFG
  • HUBMAP for level headers
• Added to Lua:
  • GT_HUB gametype
  • TOL_HUB type of level
  • hubmap field to mapheaderinfo
  • Function G_IsHubAvailable()
    • Returns a booelan.
  • Global defaulthubmap (number)
    • Cannot be set.
  • Global currenthubmap (number)
    • Set to a map number to change the current hub map.
      • Setting to 0 disables the hub.
      • Note that warping to an unlocked hub overwrites this.
  • hubmapenabled (boolean)