Merge code from cue_tag_file() and cue_tag_string().

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

With these methods a tag struct can be created
from the cdtext information in a cue sheet.
The methods depend on a cue parsing library.
Reading from strings (char*) as well as from
a file (FILE*) is supported.