Time to create a new unstable branch; stable development will now
continue in the branch v0.21.x

using the device "default" brings this plugin into line with the AlsaOutputPlugin; and a sample rate of 48kHz is more widely used as a native default for modern hardware than 44.1kHz

Also fixes an inconsistency between the docs and code.

input/plugins/AlsaInputPlugin: introduce mpd.conf config block to allow user to override the builtin defaults

input/plugins/AlsaInputPlugin: extend the alsa uri parsing to permit specification of the desired pcm audio format in the uri

This mime type is to enable the AlsaInputPlugin to communicate the pcm stream audio format to the decoder

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.

Even if the LocalStorage is available, return the "udisks://" URI when
the MapUTF8() parameter is an empty string.  This fixes the mount URI
in the state file.

Fixes a memory leak, or an assertion failure in the debug build.

Closes #487

Closes #471

Similar to commit ff1ff1e5