[applets/digitalclock] Fix timezone filter

Sometimes, the filter in the timezone selector would only
show those items already selected.

Initialize the property 'showOnlyChecked' on the proxy model
so it doesn't depend on random initialization

applets/digital-clock/plugin/timezonemodel.h

@@ -47,7 +47,7 @@ Q_SIGNALS:
 
 private:
     QString m_filterString;
-    bool m_onlyShowChecked;
+    bool m_onlyShowChecked = false;
     QStringMatcher m_stringMatcher;
 };