add knockback value to GreenSlime monster and update player transparency handling

src/de/miaurizius/jgame2d/entity/Entity.java

@@ -261,7 +261,10 @@ public class Entity {
 
         panel.player.life -= Math.max(damage, (block ? 0 : 1));
 
-        if(damage != 0) panel.player.transparent = true;
+        if(damage != 0) {
+            //setKnockback(panel.player, this, knockbackVal);
+            panel.player.transparent = true;
+        }
         panel.player.invincible = true;
     }
     public void speak() {

src/de/miaurizius/jgame2d/entity/monster/GreenSlimeMON.java

@@ -25,6 +25,8 @@ public class GreenSlimeMON extends Entity {
         exp = 2;
         projectile = new RockObj(panel);
 
+        knockbackVal = 5;
+
         solidArea.x = 3;
         solidArea.y = 18;
         solidArea.width = 42;