Fix installation for read-only files.
Fix installation for read-only files.

On Windows, we must explicitly add write permissions to the target
file before removing it, as the operation will fail if the file is not
writable.

Task-number: QBS-475
Change-Id: Ide40cc4310e327265b0a5c08f4935866762797b9
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@digia.com>

qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,villytiger/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,villytiger/qbs,villytiger/qbs,villytiger/qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,villytiger/qbs,qt-labs/qbs,gatzka/qt-labs-qbs,gatzka/qt-labs-qbs,Distrotech/qbs,qt-labs/qbs,gatzka/qt-labs-qbs,Distrotech/qbs,Distrotech/qbs,villytiger/qbs,Distrotech/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,gatzka/qt-labs-qbs,Distrotech/qbs,gatzka/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,Distrotech/qbs,gatzka/qt-labs-qbs,qtproject/qt-labs-qbs,Distrotech/qbs