Plugins can no longer emit networkpackages for *every* device.
Plugins are stored in device, wich selectively loads them.
A Device is needed in order to instantiate a Plugin (via PluginLoader)
PluginLoader is a singleton, because every instance of Device need it.
Added KPluginSelector in the KCM to select the plugins to load.
Added architecture explanation to README

Only PingPlugin is working by now.