Add .change_volume() method to Player.

This adjusts the volume by a delta and returns the new volume.
Also adjust the PlayerGUI to pull the current value from output
at each update, which should keep it synchronized with system volume
even while the popup window is open.
Tweak GUI player's track selection heuristics.

Try to ensure at least one track is selected once the playlist is
exhausted.
Shift volume control back out of subthread.

Having to wait until pauses between output makes the control very laggy
on some systems.
Add support for ID3v2 tag padding.

This ensures track.update_metadata(track.get_metadata()) round-trips
to an identical file, even if the ID3v2 tag has a bunch of unused padding
at the end.

Add lint to clean files wrapped in multiple ID3v2 tags by leaving
either one tag (for MP3/MP2/TTA) or no tags (FLAC).
Simplify ID3v2's sync-safe value handling.
Minor bugfix to track detection.