improve support for building host tools

when the build of a project requires code generators that are part of that project and are written in a compiled language, then cross-building that project requires building for two (possibly completely different) platforms.
as an extended case of this, an x-build of qt5 (and earlier) included various host tools in the installation.

this is in principle possible by setting qbs.profile in particular Products. however, the problem here is that profile names are not fixed.

to address this issue, we shall introduce 'host' and 'target' profiles which are dynamic aliases to "actual" profiles.
the pre-existing "profile" keyword on the qbs command line is renamed to "targetProfile" (and a backwards compat alias is introduced), and a new "hostProfile" keyword is introduced.
the defaultProfile setting gets the same treatment.
the immediate effect of these settings is changed to only dynamically set up the host & target profile aliases.
the qbs.profile property defaults to "target", and causes the actual instantiation of a profile.

notably, the Profile item may start using 'host' and 'target' for baseProfile, which should make projects more portable.