cda: support multiple CD audio drives

From the Add Audio CD menu item, if there are multiple drives containing audio CDs then a dialog is displayed to choose one of them.  Pressing cancel does nothing.  Selecting one and pressing OK loads all tracks from that device.  If there are no drives with CDs in then nothing happens.  If there is exactly one drive then all tracks will be loaded from it immediately.
From the command line, specifying only a "CDA file" (eg. 1.cda) loads that track number from the first device found to contain an audio CD, usually the default drive.  If the filename is not a number between 1 and 99 (eg. all.cda or 1b.cda) then all tracks will be loaded.
The device can be specificed on the command line (eg. /dev/cdrom0/all.cda).  Specifying the device without a cda filename will do nothing because it will not be understood as a cdda track.  Specifying a nonexistant device will do nothing.
From the Add Files or Add Folder menu item, selected .cda files will checked to see if they point to a CD audio device (eg. symlink, or Windows-style cda file?) and then that device will be loaded.