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