[applets/clipboard] Delete items with delete key

Pressing the delete key will now delete the highlighted list item under any of the following circumstances:

1. A list item is selected and has focus
2. A list item is selected and the search field has focus but it is empty or the insertion point is at the end of the string (in either case, pressing the delete key would otherwise do nothing)

This allows quickly and easily deleting clipboard items using only the keyboard, including after using the search field to filter it down.

BUG: 439174 FIXED-IN: 5.23

cc @teams/usability @accessibility