Updated difficulty and balance adjustments

- Added difficulty levels, easy, normal and expert
- Horse has higher stamina on normal and easy levels
- Upgrading to a tent costs 2 coins less
- Horses at stables costs 2 coin less
- Increased distance from the outer wall to the ground banner
- Adjusted costs of catapult tower to be cheaper
- When completing a scenario you go back to the welcome screen
- Fixed an issue in the siege map with the cutscene where enemy guards could spawn too close to the player.
- Reduced the number of squirrels in the forest
- Upgrade tree costs 1 coin less
- Fixed an issue where the beggar picking up a coin would not play an sfx