Adding another prairie map

- Improving AI Player to find more resource by optimizing "Auto-Exchange" feature via market(Natives) or trading post(USA)

Adding Healer units:
- Elder Squaw (Natives)
- Doctor (USA)

-Adding Prairie map for the first time (Prairie-1)

- Players will start at a random position (Playing in the same map with unknown different position.)
- More maps will be coming soon!

- Player can select difficulty: Easy, Medium, Hard

- Player can steal cows and sell them at Trading Post (USA) or Market (Natives)

- Update Fighting and Attacking system to be similar to classic RTS such as StarCraft
- Fix Box selection bug in non-1920x1080 screen resolution
- More visual and gun range on high elevation

- **Fix bug** for Bison/Cow hunting
Players can hunt bisons or cows and have workers gather their meat.

- New state bubble icons (v0.7536)

There are 3 more maps for version 0.7535

- Desert-2
- Grassland-2
- Snow-2