AllFiles iterator implementation and some small fixes

[LupertEverett]

• IsGame checks were rather broken ever since the GameLaunchInfo changes, because gameName field is now about the game's name, instead of executable. Fixed this one.
• UActor::AddChildActor() should had a nullptr check for the actor being added. Fixed this too.
• Implemented iterator function AllFiles(), which seems to be present only in Unreal 227
• GetMapName() now checks prefixes in a case insensitive manner. This fixes map filtering in Unreal Gold (the maps listed in Botmatch menu); as the maps are named like "DmAriza" instead of "DM-Ariza" like they'd in Unreal Tournament, yet the prefix to check was always "DM".