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

Some database plugins don't use a Storage (e.g. UPnP), and with this
plugin, DatabaseDetachSong() can crash.

Avoid duplicate AllocatedPath::FromUTF8() invocations in two callers.

Enables using range-based "for".

Build the UTF-8 version of the path automatically in the constructor.

Replaces map_to_relative_path() from Mapper.cxx.

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

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

Reduce global variable usage, move to frontend code.

There was quite a lot of duplicate code for loading DetachedSong
objects, with different semantics for "securely" loading local files.

Reduce bloat.