lookingGlass: Let history trim input

lookingGlass: Let history trim input

Checking whether the item is empty is now the history’s job, per the
previous two commits. The history also trims the input for us.

The effect of this is that we call _history.addItem(), and thereby move
to the end of the history, even if the input is empty (or consists only
of whitespace); clearing the input field and pressing Enter becomes a
quick way to jump back to the end of the history. (The current history
item is not overwritten if the input is empty.)

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1653>