Emit a "plugins-reloaded" signal when a new addon is installed

This ensures that the user interface will be updated to include
the new addon.

Fixes #13021.

gramps/gui/plug/_windows.py

@@ -558,6 +558,7 @@ def install_addon(self, addon_id):
         )
         pdata = self.__preg.get_plugin(addon_id)
         self.__pmgr.load_plugin(pdata)
+        self.__pmgr.emit("plugins-reloaded")
 
     def build_menu_names(self, obj):
         return (self.title, self.title)