turn off autorefresh and redraw only on demand

this should significantly reduce CPU usage but is a little bit more complicated.
while we're at it, remove the different screen redraw events and use a combined
event for all screens.

TODO:
the spotify player still needs to update the statusbar at least every second.

src/events.rs

@@ -1,14 +1,13 @@
 use crossbeam_channel::{unbounded, Receiver, Sender, TryIter};
 use cursive::{CbFunc, Cursive};
 
-use queue::QueueEvent;
 use spotify::PlayerEvent;
 use ui::playlist::PlaylistEvent;
 
 pub enum Event {
-    Queue(QueueEvent),
     Player(PlayerEvent),
     Playlist(PlaylistEvent),
+    ScreenChange(String),
 }
 
 pub type EventSender = Sender<Event>;