Use "item" terminology consistently in the UI

Most of the user interface refers to events and to-dos as "items", not
"instances".  For example, look at the dialog that "Archive Old Incidences..."
opens.

GUI:
1 job for !18 with incidence-item in 6 minutes and 46 seconds (queued for 66 minutes and 5 seconds)
latest detached
Status Job ID Name Coverage
  Build
passed #92864
linux kf5-qt5 qt5.15

00:06:46

7.0%