Commit b3a45833 added a LocateUri()
call to several playlist commands, which applied InputPlugin URI
scheme verification to playlist URIs.  This broke the SoundCloud
playlist plugin which uses "soundcloud://" URIs for which no input
plugin exists.

This commit allows the caller to specify the kind of plugin which
shall be used to verify the URI.  Right now, only "input" is
implemented; "storage" uses the "input" verification for now; and
"playlist" has no verification at all (for now).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/528

Closes https://github.com/MusicPlayerDaemon/MPD/issues/234

implement for the "load" command the same logic used for the "add"
command: local clients can load playlist specified as absolute paths.

For relative paths the old logic is preserved: first look for a stored
playlist, then look in the music directory.

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.