Added difficulty screen

Signed-off-by: Hans Kokx <hans.d.kokx@gmail.com>

lib/classes/difficulty.dart

@@ -0,0 +1,12 @@
+enum Difficulty {
+  canIPlayDaddy(0, "Can I play, Daddy?"),
+  dontHurtMe(1, "Don't hurt me."),
+  bringEmOn(2, "Bring em' on!"),
+  iAmDeathIncarnate(3, "I am Death incarnate!"),
+  ;
+
+  final String title;
+  final int level;
+
+  const Difficulty(this.level, this.title);
+}