Avoid side effects during menu initialization

Summary: Setting some attributes, like visible, enabled, etc has side effects. Do not set them if these are not changed.

Test Plan: Without this change menus (mostly submenus) randomly shows up when the SNI is updated, e.g. every time VLC changes a track I get its "speed (slower, normal, faster)" menu open.

Reviewers: #plasma, #frameworks, broulik

Reviewed By: #plasma, broulik

Subscribers: nicolasfella, davidedmundson, cgiboudeaux, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25223