ContextAlbum: Delete timeline to delete previous cover

QTimeLine was holding the previous covers shared pointer in the signal/slot connection, which caused it to never be free'd even though it's removed from the previous_covers_ list.
To fix this, make sure the QTimeLine is deleted.

This fixes a huge memory leak.

Addresses issue #1464

src/context/contextalbum.cpp

@@ -235,6 +235,7 @@ void ContextAlbum::FadePreviousCover(SharedPtr<PreviousCover> previous_cover) {
 
 void ContextAlbum::FadePreviousCoverFinished(SharedPtr<PreviousCover> previous_cover) {
 
+  previous_cover->timeline.reset();
   previous_covers_.removeAll(previous_cover);
 
 }