applets/clipboard: Don't delete any item when the search field has focus

The behavior may cause unexpected results when the user spam press Del
in order to delete all characters in the search field, and find that
all items in the list are deleted.

It also causes pressing Del in the search field not deleting the fully
selected text.

BUG: 448738
BUG: 448739


(cherry picked from commit 79344a3b)