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.

Eliminate yet another access to the global ConfigData instance.

Eliminate a use of GetGlobalConfig().

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.

release v0.20.21

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

This was missing in commit 8266ab55 for #69.

The number of MPD installs on Android < 5.0 is negligible, and that
API version introduces lots of useful features for MPD.

Prevent Meson from running Android-x86 binaries.  That will fail
because the Android standard libraries are most likely not installed.

The Curl plugin requires libexpat in order to work these days, so we should
download and build it in order to get the plugin enabled on Windows.