Since the brightness plugin merged with the battery plugin,
rename it to Power Manager Plugin. Also since we support
LXDE we'll stop calling it the Xfce Power Manager Plugin as
a LXDE plugin.

Thanks to Simon for pointing out that a custom menu item would
accomplish what we need and for pointing out the ubuntu app
indicator code that this patch was based on. Additional thanks
need to go to Cody Russell for creating a custom menu slider
widget I could copy.