iPod collection: convert from ReplayGain to Sound Check and vice versa

By popular demand, we now convert ReplayGain values to Sound Check
values when transferring tracks to iPod. We also read Sound Check from
iPod and convert it to ReplayGain on-the-fly when playing a track off
an iPod in Amarok.

On the other hand, when transferring tracks from iPod to Amarok, Sound
Check from iPod is _not_ automatically copied (current limitation of
the ReplayGain implementation in Amarok), but Amarok picks up
ReplayGain information from meta-data stored inside the track. (Does
anyone know whether iTunes save Sound Check somewhere in song
meta-data?)

It is currently hard-coded that *track* ReplayGain is used (Sound Check
is not able to differentiate between track and album gains), this could
be configurable in future (but hey, micro-options!) or it could respect
Settings -> Replay Gain Mode (tricky, the setting would be read *only
when the song is being transferred to iPod*)

FEATURE: 142579
FIXED-IN: 2.6
DIGEST: volume normalization now works between Amarok and iPods