[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: 5.78