Implement custom progress indicator and seeking

Squashed commit of the following:

commit 23b9a3deac564bf3753a00238784a6045cb50d46
Author: Arne Schlüter <arne@schlueter.is>
Date:   Sun Oct 14 10:20:08 2018 +0200

    Enable seeking in buffered part and fix drawn x value

commit 9ce4b0941f4a57286f608d2b155658672cac3817
Author: Arne Schlüter <arne@schlueter.is>
Date:   Sun Oct 14 09:40:43 2018 +0200

    Draw seek position and enable seeking played part by click

commit 58cbf2d8035c0eeacaed3da7a68f97d94db4a2b6
Author: Arne Schlüter <arne@schlueter.is>
Date:   Thu Oct 11 21:42:57 2018 +0200

    Add retina canvas

commit 6acb84a67e4bee61e5b9ae6eb15e8159e0431662
Author: Arne Schlüter <arne@schlueter.is>
Date:   Wed Oct 10 17:52:43 2018 +0200

    Implement canvas progress bar

src/cljs/airsonic_ui/audio/core.cljs

@@ -14,11 +14,13 @@
   "Takes an audio object and returns a map describing its current status"
   [elem]
   {:ended? (.-ended elem)
-   :loop? (.-loop elem)
-   :muted? (.-muted elem)
    :paused? (.-paused elem)
    :current-src (.-currentSrc elem)
-   :current-time (.-currentTime elem)})
+   :current-time (.-currentTime elem)
+   :seekable (let [seekable (.-seekable elem)]
+               (if (> (.-length seekable) 0)
+                 (.end seekable (dec (.-length seekable)))
+                 0))})
 
 ; explanation of these events: https://developer.mozilla.org/en-US/Apps/Fundamentals/Audio_and_video_delivery/Cross-browser_audio_basics
 
@@ -60,6 +62,12 @@
        (.play a)
        (.pause a)))))
 
+(re-frame/reg-fx
+ :audio/seek
+ (fn [[percentage duration]]
+   (set! (. @audio -currentTime)
+         (* percentage duration))))
+
 ;; subscriptions
 
 (defn summary