Control default install root from project files

Currently qbs.installRoot property is read-only.

As a qml plugin developer I would like to set a custom install root (like using equivalent of qmake QT_INSTALL_QML var to install system wide) to deploy my product without the user having to copy it from the default installRoot dir or setting a qbs command line argument.

It used to be possible with a relative path trick but not anymore since commit 779de6d.

So please allow to set qbs.installRoot from QBS files.