Re-introduce property qbs.installPrefix.

This property used to exist and was erroneously removed when the
"install root" parameter was introduced. The author has now learned that
"install prefix" and "install root" are actually orthogonal concepts.

Change-Id: I8f704c92d09817063b82a175693899af9adc9bac
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>