[installer][target] setup extraction progress. Some refactoring

Ok, thread is necessary to get proper gui updates on extraction. Therefor the code needed further refactoring so we trigger the right steps only when we finished extraction (modularizing, at least in methods is way more readable by the way).

At least I think my approach is the only sane quick-shot solution we can get with QT4.8.
Check it out.
I've used an extra tar.xz for my local tests.
Created using dd if=/dev/zero of=dummy bs=1g count=2 && tar cJf dummy.tar.xz dummy
(you may need bs=1G) this creates a wonderful 2G file which gives some action...have fun!