Signed-off-by: FlashSystems <developer@flashsystems.de>

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

The new class is pluggable, to prepare for gzipped database files.

For now, the TextFile class remains, and will be refactored away
later.

Use Storage::MapUTF8() internally, don't use global variables.

Move from db/DatabaseGlue.cxx, eliminating global variable.

Fixes loading database songs from state file (regression by commit
29072797).

Another big chunk of code for multi-player support.