WelcomePage: Reintroduce fallback copying.

In case the project file is not user-writable,
assume that the entire project isn't and create
a copy. Useful e.g. for distro packaged examples.

Also, make the additonal files to open work properly.

Change-Id: Iedd590ba7709c0e689adcb3944e75394b6ef0955
Reviewed-on: http://codereview.qt.nokia.com/1097


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>