dbus API: make CurrentSong output more fields (Closes #61)
move mostly identical code from Songs::Set and Songs::UpdateTags to a new function
add function Gtk2::Dialog::add_button_custom
change arguments to CopyMoveFiles() function

fixes error dialogs not appearing above the Mass Rename dialog
small code clean-ups
various improvements and small fixes to error dialogs asking to retry
add auto-complete button to disc and year in mass-tagging dialog