viewSelector: Don't focus search entry on whitespace

viewSelector: Don't focus search entry on whitespace

We recently started to trim leading and trailing whitespace from
the search string, and not to trigger a search when the resulting
string was empty. However we still allow whitespace to trigger
type-ahead-find, so that the key focus is moved briefly to the
search entry and back to the stage, resulting in a disruptive
flickering of the entry.
Fix this by excluding whitespace from triggering type-ahead-find.

https://bugzilla.gnome.org/show_bug.cgi?id=694475