Use xfce4-panel debugging methods

Use xfce4-panel debuging methods described on
http://docs.xfce.org/xfce/xfce4-panel/debugging#debugging_plugins.
In particular log to stderr and enable debug logging if the PANEL_DEBUG
environment variable is set to "all".
Change G_LOG_DOMAIN to the more intuitive pulseaudio-plugin.
Remove logging to a logfile as multiple plugin instances clobber the
logfile and automatic restart after abort() or crash truncates the
file.
Use pulseaudio_plugin_debug instead of g_debug which provides
additional information.

Conflicts:
	panel-plugin/pulseaudio-plugin.c