Utils: Use a bit more Key and Store

Also, separate the Key into a separate header to minimiz impact
on #include load.

Change-Id: I4f719cad4d23a71a9c228d0b7f5c2409fd6e24d5
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

src/plugins/coreplugin/foldernavigationwidget.cpp

@@ -841,7 +841,7 @@ void FolderNavigationWidgetFactory::saveSettings(Utils::QtcSettings *settings,
 {
     auto fnw = qobject_cast<FolderNavigationWidget *>(widget);
     QTC_ASSERT(fnw, return);
-    const QString base = kSettingsBase + QString::number(position);
+    const Key base = kSettingsBase + Key::number(position);
     settings->setValueWithDefault(base + kHiddenFilesKey,
                                   fnw->hiddenFilesFilter(),
                                   kHiddenFilesDefault);