Closes #360

Closes #366

In this new order, a filter 'AlbumArtist ""' matches only on songs
which neither have `AlbumArtist` nor `Artist`.

Grouping in the "list" command was completely broken from the start,
unlike "count group".  I have no idea what I have been thinking when I
wrote commit ae178c77, but it didn't
make any sense.

This commit is a rewrite of the feature.

For clients to be able to detect this feature, this commit also
increments the protocol version.

Be consistent with "list" responses.

Just like AlbumArtist falls back to Artist, AlbumArtistSort should
fall back tom AlbumArtist, ArtistSort and finally Artist.

Closes #355

This improves the workaround from commit
b5ba94f1 and actually gives a useful
result for "list" with a disabled tag.

Sometimes, the JVM returns a non-nullptr value with an exception
pending (seen on Android 1.6, maybe a Dalvik bug?).  Let's catch all
such cases.

MPD has minSdkVersion=21 which is above all the checks here.

Since nfs_stat_async is deprecated.

This is the version in Debian Jessie (oldstable), a reasonable "old
enough" version to keep support for.

add 2 preferences to:
 - enable Wakelock when MPD is running (prevent suspend)
 - run MPD on boot

and display MPD logs

 - <uses-permission> must be before <application>
 - specify allowBackup (default)

- add Settings: Activity to start / stop MPD Service (Main).

- Main is a service that run in foreground with a notification. See
  Service.startForeground documentation for more details.

- Main.Client is used to control the service: start or stop it and also receive
  callbacks when service encounters an error, is killed, is started or is
  stopped.

- Main.start to start the service without any fallback.

A Java object to send logs on the android side.

javah is deprecated.

When using a database that was not created with a WebDAV music_directory
(i.e., if using a remote database, on which updates happen locally) and
using the Curl storage plugin, MPD would previously send GET requests that
had unescaped spaces in them.  This change uses Curl's URL-encode API to
solve this.

`$srcdir/.git` doesn't exist if `$srcdir` is a worktree.

This was missing in commit 8266ab55 for #69.