Introduce and use FileName::exists()

This can use the faster route through QFileInfo::exist now.

Change-Id: Idb41b5d5185d7f02eacba498fb01f483d95e8d57
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Reviewed-by: Christian Stenger <christian.stenger@digia.com>

src/plugins/cmakeprojectmanager/cmakeproject.cpp

@@ -586,7 +586,7 @@ QString CMakeProject::uiHeaderFile(const QString &uiFile)
     while (baseDirectory.isChildOf(project)) {
         Utils::FileName cmakeListsTxt = baseDirectory;
         cmakeListsTxt.appendPath(QLatin1String("CMakeLists.txt"));
-        if (cmakeListsTxt.toFileInfo().exists())
+        if (cmakeListsTxt.exists())
             break;
         QDir dir(baseDirectory.toString());
         dir.cdUp();