Fixed|Hexen: Record Variable 'songLump' not found

Removed obsolete logic for the Hexen specific, "currentMap" handling
Updated the SndInfoParser to modify the relevant Music definition
directly (Hexen uses separate Music definitions for each map).

IssueID #1910
Audio|Client: Try other music sources if playback fails

Although the preferred source may be available playback may still
fail for some reason (e.g., the preferred source is CD audio yet
the specified track number does not exist).

If playback does fail, then continue to try all sources according
to the user's preferred order.
Merge branch 'master' of github.com:skyjake/Doomsday-Engine