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

Prepare to convert "partition" to a mutable pointer.

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

playlist_file_print() already handles stored playlists.  spl_print()
is never called with a successful result.

Wrapper for protocol/ArgParser.cxx.

Implement a "bulk" edit mode that postpones both UpdateQueuedSong()
and OnModified().  This way, the playlist version gets incremented
only once.  More importantly: when adding multiple songs to a queue
that consists of only one song, the first song that got added will
always be played next.  By postponing this choice, all newly added
songs get a chance to become the next song.  Fixes the second (and
last) part of Mantis ticket 0004005.

This case is handled already by playlist_open_in_playlist_dir() (via
playlist_mapper_open()).  And the call didn't work anyway.

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.

Playlist file name first, to be consistent with the other functions in
this library.

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