Fix the Optional Features menu

This is a backport of Bitcoin-ABC@ff1a101

After commit 1fa9c81, `descr["__name__"]` no longer matches the short name key expected by `plugins.get_internal_plugin(name)`.
As a result, all plugins in that menu dialog were greyed out, and the user could no longer disable or enable a plugin.

For existing users, everything stayed the same (previously enabled plugins stayed enabled and vice-versa), so the damage is mainly limited to new users not being able to enable CashFusion.

electroncash_gui/qt/main_window.py

@@ -5152,7 +5152,9 @@ def do_toggle(weakCb, name, i):
                 run_hook('init_qt', gui_object)
 
         for i, descr in enumerate(plugins.internal_plugin_metadata.values()):
-            name = descr['__name__']
+            # descr["__name__"] is the fully qualified package name
+            # (electroncash_plugins.name)
+            name = descr["__name__"].split(".")[-1]
             p = plugins.get_internal_plugin(name)
             if descr.get('registers_keystore'):
                 continue