Supply media button images, instead of using whatever's on the device.

This ensures that the play/pause/next/previous button images are consistent
with those for shuffle and repeat, and don't change depending on the API
level of the device Squeezer is installed on.

The images are from http://developer.android.com/design/downloads/index.html.
Ignore the squeezer.properties file.

This file contains passwords and signing keys, and should never be
submitted to the repository.