preferences: Move the plugins settings

So, I hope it will suit you.

However, I will need your guidance before it's complete....

First, because of the plugins list, the window take too much space. It can be fixed by settings the window resizable but I don't know if it's what you want.

Secondly, the list is not sorted. peas_engine_get_plugin_list returning a const GList* I don't know if it's a good idea to remove the cast with g_list_sort((GList*)plugins_list, ...). Libpeas is pretty obscure to me I don't know what it does with the list in the background.

Closes: #548 (closed)

Thanks