Generalize the target setup page and move it into projectexplorer

Move the qmake specific code into a projectimporter class with
a specialization for qmake projects in the qt4projectmanager.

This change depends heavily on the BuildConfigurationFactory cleanups
done earlier and completes that change in such a way that generic
build configuration factories are now in theory possible. The
remaining problem is how to select the best factory of several that
claim to be able to handle a kit and that is left for the next patch.

Change-Id: I47134cb1938c52adebcdc1ddfe8dbf26abbbbeee
Reviewed-by: Daniel Teske <daniel.teske@digia.com>