#530 Enable cpufreq setting

[Charles Lehner]

I updated the cpufreq lxpanel plugin to allow for setting the cpu frequency/governor instead of just viewing it. This functionality was already in the plugin but was disabled because it requires root privileges. The way I handle this is to spawn gksu to chown the appropriate sysfs files to the current user when needed. I also updated the menus to use radio buttons to show the current selection.

I realize the approach of using gksudo won't work if lxpanel is running on a system that does not have gksudo. Should something else be done in that case?