Code review comment for lp:~gary-wzl77/unity-scopes-shell/fix_1576311

Actually, we need to trigger this signal.
I take unity-scope-tool for example.
If we don't emit searchQueryChanged() signal in following case
  1.query sth by typing some keywords
  2.click 'Cancel'
The textedit of search query will not be updated, which should be cleared up after a query is cancelled.