all: Use "desktop-entry" hint
None of the plugins that use notifications seem to set the useful "desktop-entry" hint. This hint is useful to tie a system notification to a Settings panel. No system notification should appear that isn't tied to a desktop entry and therefore a UI.
Clicking on the notification in the Calendar shell should take the user to that "desktop entry", so to the application that's most likely to help with attending to the problem.
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