After a discussion with Mattias Clasen and exploring alternative
options, we decided to switch back to GtkImageMenuItem as it works
best for what we want here.
In order for people to be able to compile without warnings, the
relevant parts have been wrapped in G_GNUC_BEGIN_IGNORE_DEPRECATIONS.

Borrowed from upstream libido, thanks to Lars Uebernickel.

This change was ported from libido upstream:
http://bazaar.launchpad.net/~indicator-applet-developers/ido/trunk.15.04/revision/187

Also no more deprecated warnings.

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.