Add a real playlist (#20)

* Start implementing playlist

Done so far:
* Creation
* Changing playback mode
* Changing repeat mode

* Add skipping for linear playlists with all repeat modes

* Complete implementation for playlist/next-song

* Implement all the playlist skipping functionality

* Add functions to enqueue songs

* Remove start-idx parameter when constructing playlists

* Use namespaced keywords only when modifying external data

E.g. songs in the queue

test/cljs/airsonic_ui/utils/helpers_test.cljs

@@ -0,0 +1,14 @@
+(ns airsonic-ui.utils.helpers-test
+  (:require [cljs.test :refer [deftest testing is]]
+            [airsonic-ui.utils.helpers :as helpers]))
+
+(deftest find-where
+  (testing "Finds the correct item and index"
+    (is (= [0 1] (helpers/find-where (partial = 1) (range 1 10))))
+    (is (= [2 {:foo true, :bar false}] (helpers/find-where :foo '({}
+                                                                  {:foo false
+                                                                   :bar true}
+                                                                  {:foo true
+                                                                   :bar false})))))
+  (testing "Returns nil when nothing is found"
+    (is (nil? (helpers/find-where (partial = 2) (range 2))))))