Add a runtime debugging mode to both the mixer application and the panel plugin
which will print additional diagnostic output.
Add a "--debug" command line option to xfce4-mixer which will activate the
debugging mode.
Make the panel plugin look for the PANEL_DEBUG environment variable containing
"mixer" in order to enable the debugging mode.
Add custom xfce_mixer_debug() function for printing debugging output.
Replace DBG() macro with xfce_mixer_debug().
Print relevant GStreamer data on startup when in debugging mode.
Add "--version command" line option to xfce4-mixer which will print the
version.