Path to xfce4-dockbarx-plug should not be hardcoded

[TiZ-HugLife]

trafficstars

Okay, so I did a thing between 0.2 and 0.2.1 where I moved the install location of the plug into /usr/share/xfce4/panel/plugins. I originally intended for that location to be configurable, which is evidenced by wscript in the initial commit. But I never could figure out how to get Vala defines to work in waf, so I just hardcoded it.

This should be fixed, but it's not like the plugin will work in any prefix other than /usr/share anyways, so it doesn't matter a whole lot.

[TiZ-HugLife]

18.04 broke stuff so now I basically have to do this.