Plugins maybe want to use in their activation Gajim methods that
use an extension point. But because the PluginManager Object is not fully
initialized, the extension point method is not available yet
First create the PluginManager Object, then load Plugins.
Remove plugin path from sys.path after importing.
If we scan multiple plugin dirs and never remove the path from
sys.path, it is hard to predict from which path python will import
a package.
The chance that the same plugin package is in multiple plugin paths
is high.
