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.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information