Dont activate plugins in PluginManager init

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.

gajim/gui_interface.py

@@ -2761,6 +2761,7 @@ class Interface:
         # Creating plugin manager
         from gajim import plugins
         app.plugin_manager = plugins.PluginManager()
+        app.plugin_manager.init_plugins()
 
         helpers.update_optional_features()
         # prepopulate data which we are sure of; note: we do not log these info

gajim/plugins/pluginmanager.py

@@ -110,7 +110,6 @@ class PluginManager(metaclass=Singleton):
         for path in [app.PLUGINS_DIRS[1], app.PLUGINS_DIRS[0]]:
             pc = PluginManager.scan_dir_for_plugins(path)
             self.add_plugins(pc)
-        self._activate_all_plugins_from_global_config()
 
     @log_calls('PluginManager')
     def _plugin_has_entry_in_global_config(self, plugin):
@@ -126,6 +125,10 @@ class PluginManager(metaclass=Singleton):
     def _remove_plugin_entry_in_global_config(self, plugin):
         app.config.del_per('plugins', plugin.short_name)
 
+    @log_calls('PluginManager')
+    def init_plugins(self):
+        self._activate_all_plugins_from_global_config()
+
     @log_calls('PluginManager')
     def add_plugin(self, plugin_class):
         '''