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.cpp

@@ -190,7 +190,7 @@ void Engine::Base::ReloadSettings() {
 
 }
 
-void Engine::Base::EmitAboutToEnd() {
+void Engine::Base::EmitAboutToFinish() {
 
   if (about_to_end_emitted_) {
     return;