[AFR] Implementing Class enchantments (ready for review) (#7992)

* [AFR] Implemented Druid Class

* [AFR] Implemented Wizard Class

* [AFR] Implemented Cleric Class

* [AFR] Implemented Fighter Class

* reworked class ability implementation

* fixed an error with setting class level

* small reworking of class triggers

* added class level hint

* added tests

* small change

* added common class for reminder text

Mage/src/main/java/mage/game/events/GameEvent.java

@@ -340,6 +340,7 @@ public class GameEvent implements Serializable {
          */
         BECOMES_EXERTED,
         BECOMES_RENOWNED,
+        GAINS_CLASS_LEVEL,
         /* BECOMES_MONARCH
          targetId    playerId of the player that becomes the monarch
          sourceId    id of the source object that created that effect, if no effect exist it's null