applets/notifications: Only expand canonical plasmoid on showNotificationsRequested

Ameen Al-Asadyrequested to merge
ameenalasady/plasma-workspace:fix-notif-show-canonical-only into master

With a panel+tray per monitor, clicking Show on the DND missed-notifications summary expanded history on both screens, flashing on primary then sticking on secondary. Only expand Globals.plasmoidItem so it opens once where the summary was. Tested on CachyOS Plasma 6.7.4 Wayland dual-monitor.

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