Lists files and directories.  Supports storage plugins.

A SimpleDatabase instance can now "mount" other Database instances at
certain locations.  This is used to use a new SimpleDatabase instance
for each storage mount (issued with the "mount" protocol command).
Each such instance has its own database file, stored in the directory
that is specified with the "cache_directory" option.

Add the new idle event to Idle.hxx/Idle.cxx.

Implements the error checks missing in commit 0bad8406

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.

Move global variables into the new classes.  That may allow multiple
update threads for multiple databases one day.

Decouple command/OtherCommands.cxx from the "simple" database plugin.

Don't depend on the "simple" database plugin.  This fixes an assertion
failure / crash and allows using stickers with other plugins.

Another big chunk of code for multi-player support.

This commit adds the NeighborPlugin API which can be used to detect
nearby file servers that can be used by input plugins.  This list of
servers is exported using the new "listneighbors" command.  The idle
even "neighbor" notifies interested clients when a new neighbor is
found or an existing one is lost.

There's a lot missing currently: protocol&user documentation, and a
way to "mount" remote servers into the music database.  Obviously,
some code from the UPnP database plugin can be moved to a neighbor
plugin.