Add a delete button to notifications

A magyar leírás lent található

## English description

A request came that it'd be nice to be able to delete the notifications. There should be an 'X' next to every notification item that deletes it from the list.

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## Magyar leírás

Jött egy kérés, hogy lehessen értesítéseket egyesével törölni. Ehhez kéne egy "X" gomb minden értesítéshez a felugró menüben és az aloldalon is.

Start by locating how notification items are rendered in the popup menu and the notifications subpage. Trace the existing notification data flow before deciding where deletion belongs. Done means each notification has an X control and can be removed from both views.

Written by the indexing model from the issue text.