[Enhancement] Implement frequency/governor changes

Changes are executed via cpufreq-service - a dbus service which responds to a cpufreq-plugin dbus message & exits, so no wasted resources.

The UI also got a face-lift to reflect new changes and allow changing the settings for "All CPUs".

The Wiki should also be changed to reflect this enhancement, as it currently states: At the moment, changing any of these settings is not supported by this plugin but might be implemented in future releases

Also in the Wiki, Known bugs section can be removed.

This also closes #1.