introduce /get property variants

introduce /get property variants

properties are now split into a write location $$[FOO] and a read
location $$[FOO/get]. the write locations are hard-coded and configurable
via qt.conf/Paths as before, while the read locations are configured via
qt.conf/EffectivePaths.

this finally provides a clean solution to the problem that during the qt
build itself tools and libraries need to be taken from somewhere else
than they are installed to.

Change-Id: I956c43bd082afd465e690fe75d0bee3c2c0f7c25
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>