Improve UI of the search edit.

- The copy action (Control+C) when no text is selected copies the
  password of the current entry.  This should be reasonable when
  Control+B copies the username.

- Down at EOL moves the focus to the entry view.  Enter and Tab should
  do that, but it would be handy for user to be able to get to the third
  entry by hitting Down three times.

src/gui/DatabaseWidget.h

@@ -102,6 +102,9 @@ Q_SIGNALS:
     void splitterSizesChanged();
     void entryColumnSizesChanged();
 
+protected:
+    bool eventFilter(QObject* object, QEvent* event);
+
 public Q_SLOTS:
     void createEntry();
     void cloneEntry();