Qt Version refactoring

Split up target specific code into subclasses. Also change
Qt4BuildConfiguration to allow a null qtversion.

Remove code that relied on always having a qt version.
Also make it possible to remove all qt versions.

Completly change the qt in path autodetection to be only
a fall back if no configuration was found.

Note: For now the old settings are not removed, as such 2.2 and master
can coexist.

Reviewed-By: hunger