Fix Last.fm scrobbling after seek

Fixes #4836
Last.fm defines a scrobble should be sent if:
-the track is longer than 30 seconds.
-the track has been played for at least half its duration, or for 4 minutes (whichever occurs earlier.)

Clementine has treated this as seconds from the start of the track, and if any seeking occurs, it nullifies the scrobble.
This IMO is incorrect. If I skip the first 10 seconds of a song, but listen to the rest (still meeting the time requirements),
I should still be able to scrobble the play. This change moves the scrobble point with every seek, requiring continuous playback
from any point that satisfies the time criteria.

src/core/player.cpp

@@ -436,13 +436,9 @@ void Player::SeekTo(int seconds) {
       qBound(0ll, qint64(seconds) * kNsecPerSec, length_nanosec);
   engine_->Seek(nanosec);
 
-  // If we seek the track we don't want to submit it to last.fm
-  qLog(Info) << "Track seeked to" << nanosec << "ns - not scrobbling";
-  if (app_->playlist_manager()->active()->get_lastfm_status() ==
-      Playlist::LastFM_New) {
-    app_->playlist_manager()->active()->set_lastfm_status(
-        Playlist::LastFM_Seeked);
-  }
+  // If we seek the track we need to move the scrobble point
+  qLog(Info) << "Track seeked to" << nanosec << "ns - updating srobble point";
+  app_->playlist_manager()->active()->UpdateScrobblePoint(nanosec);
 
   emit Seeked(nanosec / 1000);
 }

src/playlist/playlist.cpp

@@ -1705,14 +1705,25 @@ Song Playlist::current_item_metadata() const {
   return current_item()->Metadata();
 }
 
-void Playlist::UpdateScrobblePoint() {
+void Playlist::UpdateScrobblePoint(qint64 seek_point) {
   const qint64 length = current_item_metadata().length_nanosec();
 
-  if (length == 0) {
-    scrobble_point_ = 240ll * kNsecPerSec;  // 4 minutes
+  if (seek_point == 0) {
+    if (length == 0) {
+      scrobble_point_ = 240ll * kNsecPerSec;  // 4 minutes
+    } else {
+      scrobble_point_ =
+          qBound(31ll * kNsecPerSec, length / 2, 240ll * kNsecPerSec);
+    }
   } else {
-    scrobble_point_ =
-        qBound(31ll * kNsecPerSec, length / 2, 240ll * kNsecPerSec);
+    if (length == 0) {
+      // current time + 4 minutes
+      scrobble_point_ = seek_point + (240ll * kNsecPerSec);
+    } else {
+      scrobble_point_ =
+          qBound(seek_point + (31ll * kNsecPerSec), seek_point + (length / 2),
+                 seek_point + (240ll * kNsecPerSec));
+    }
   }
 
   set_lastfm_status(LastFM_New);

src/playlist/playlist.h

@@ -227,6 +227,7 @@ class Playlist : public QAbstractListModel {
   }
   void set_lastfm_status(LastFMStatus status) { lastfm_status_ = status; }
   void set_have_incremented_playcount() { have_incremented_playcount_ = true; }
+  void UpdateScrobblePoint(qint64 seek_point = 0);
 
   // Changing the playlist
   void InsertItems(const PlaylistItemList& items, int pos = -1,
@@ -351,7 +352,6 @@ signals:
 
  private:
   void SetCurrentIsPaused(bool paused);
-  void UpdateScrobblePoint();
   int NextVirtualIndex(int i, bool ignore_repeat_track) const;
   int PreviousVirtualIndex(int i, bool ignore_repeat_track) const;
   bool FilterContainsVirtualIndex(int i) const;