viewer: ensure view is updated if the cache is shifted

Also moves "ForceUpdate" to a queued connection so it occurs as soon as
the rest of the processing is done. This ensures the viewer isn't updated
too early.

app/render/playbackcache.cpp

@@ -82,7 +82,11 @@ void PlaybackCache::Shift(const rational &from, const rational &to)
     locker.unlock();
 
     emit Invalidated(invalidate_range);
+  } else {
+    locker.unlock();
   }
+
+  emit Shifted(from, to);
 }
 
 void PlaybackCache::Validate(const TimeRange &r)