Fix and improve gapless playback

If "about-to-finish" was emitted before the preload time was reached, we never set the next uri, so gapless playback was broken.
Make sure to always set the next uri, and increase preload gap from 5 to 8 seconds.

src/engine/enginebase.h

@@ -102,9 +102,7 @@ class Base : public QObject {
  public slots:
   virtual void ReloadSettings();
   void UpdateVolume(const uint volume);
-
- protected:
-  void EmitAboutToEnd();
+  void EmitAboutToFinish();
 
  public:
 
@@ -217,7 +215,6 @@ class Base : public QObject {
   bool http2_enabled_;
   bool strict_ssl_enabled_;
 
- private:
   bool about_to_end_emitted_;
   Q_DISABLE_COPY(Base)