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>