When running some versions of UPower, the code path had a minor
logic error causing it to never return the "display device". This
patch fixes that check.

Only build in the new xfce_dialog_show_help_with_version function
if the system has libxfce4ui 4.11.1, otherwise use the old help
function in order to still support systems with Xfce 4.10.

The LXPanel plugin API was completely changed in version 0.7.0
without any module versioning or backwards compatibility. This
patch make the plugin work with the new version while retaining
the ability to still build for the old version (based on checking
the package version at compile time).

share/xfce4/panel-plugins has been deprecated in favor of
share/xfce4/panel/plugins.

There are a couple conditions where the plguin would crash or
emit runtime warnings due a device being connected or
disconnected. Those should be fixed now.

xfpm now points to the versioned docs hosted at docs.xfce.org.
This version string will always point to the major.minor of xfpm
so it will auto-increment in future relases.

When the xfconf brightness-slider-min-level is set, the plugin
will now dynamically update the range slider (if shown) for the
new minimum as well as stopping the mouse wheel from scrolling
below that level.

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.