Draft: Another idea against volume spikes when ReplayGain changes from low to high

Gain was adjusted in slotNewTrackPlaying, which will be called only after the new track has started playing, causing a volume spike of a second or so. Try to avoid this by only doing gapless playback when the next track is from the same album, and by setting gain a bit earlier in playback preparation if possible.

Shouldn't affect any cases where ReplayGain is not available (= phonon-vlc) or used.

BUG: 299461

Previous attempt was not good enough, now testing this one.