history: Trim input and ignore if empty

history: Trim input and ignore if empty

This ports the runDialog changes of [1] to the underlying history
component, where they can benefit looking glass as well: the history is
now responsible for trimming the input and deciding that it shouldn’t be
stored if empty. (Note that _setPrevItem and _setNextItem already
skipped updating the history if the entry was empty.)

Since both users, runDialog and lookingGlass, also need the trimmed
input for other reasons – runDialog to avoid issues when interpreting
the command as a file path (if it can’t be executed as a command),
lookingGlass to decide whether a command should be run at all – have
addItem return the trimmed input. (runDialog and lookingGlass are not
yet changed to take advantage of this – that will be done in separate
commits.)

[1]: https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1442

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