De-inline QFileSystemModel::fileInfo() and implement it efficiently

De-inline the method, and return the file info that's already being
cached by the model.

This is ok to do in a minor release, as apps compiled against an older
version of Qt will simply continue to use the less efficient
implementation.

Change-Id: I164c7961a8cf97447638af316512326442767dd8
Task-number: QTBUG-30902
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Edward Welbourne <edward.welbourne@theqtcompany.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>