support command line argument transformation

qbs projects are highly configurable through the properties of the modules they use. however, users are used to configure scripts which offer a more "natural" and less overwhelming view on the the possible build options.
to support such a use case, projects should be able to define transformer objects which take arguments from qbs' command line and transform them to properties, and add a short help text.
qbs could then be invoked this way:

# configure+build
qbs --prefix=/opt/foobar --without-baz someproperty:value -- -ambiguous-option
# get help for the aliases defined by the project (and qbs itself, more at the bottom)
qbs -help