Improvements for json validation for mods

- Implemented validation for `targetCondition` in spells
- Implemented validation for `mapObject` in towns/heroes
- Fixed validation of zone connections in RMG
- Added workarounds to prevent assertions triggering on invalid mods
- Erase 'base' entries from json before validation (but after applying
them to derived keys)

Should have no effect on mod behavior/support, but may cause new
detections for mods that were broken in either 1.6 or 1.7

lib/CSkillHandler.cpp

@@ -316,6 +316,8 @@ void CSkillHandler::beforeValidate(JsonNode & object)
 	inheritNode("basic");
 	inheritNode("advanced");
 	inheritNode("expert");
+
+	object.Struct().erase("base");
 }
 
 std::set<SecondarySkill> CSkillHandler::getDefaultAllowed() const