Tweak menus a bit, add proper single player menu, made back link have some margin from above links

lib/ui/menus/single_player_menu.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+class IMICFPS
+  class SinglePlayerMenu < Menu
+    def setup
+      title I18n.t("menus.singleplayer")
+
+      link "Tutorial", enabled: false, tip: "No tutorial implemented, yet..."
+
+      link "Campaign", enabled: false, tip: "No campaign, yet..."
+
+      link "Multiplayer Practice" do
+        push_state(LevelSelectMenu)
+      end
+
+      link I18n.t("menus.back"), margin_top: 25 do
+        pop_state
+      end
+    end
+  end
+end