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/libs/utils/fileutils.h

@@ -72,6 +72,7 @@ public:
     static FileName fromUserInput(const QString &filename);
     QString toString() const;
     QString toUserOutput() const;
+    bool exists() const;
 
     FileName parentDir() const;