Plugin threading

This PR intends to fix issue #428
Besides the two mentioned plugins I also found chk_poison blocking the UI.

In src/interfaces/gtk/ec_gtk_plugins.c:
```c
   /*
    * pay attention on this !
    * if the plugin init does not return,
    * we are blocked here. So it is encouraged
    * to write plugins which spawn a thread
    * and immediately return
    */
   if (plugin_is_activated(plugin) == 1)
      plugin_fini(plugin);   
   else
      plugin_init(plugin);
```
However I found yet no better way than just returning in plugin state finished before the plugin acutally finished it's work.