Use  git submodules for Qt Base and Qt WebKit

As discussed on the mailing list this move greatly decreases the maintenance cost associated with updating Qt Base or Qt WebKit.

For now, the submodules point to the KDAB owned forks of both, Qt Base and Qt Webkit, but we would appreciate if the PhantomJS could take these over and create official forks.

Both Qt Base and Qt WebKit forks are based on snapshots of the 5.3 branches upstream from around Aug. 12th.

Additional to the submodules change, this pull request contains the patch to add a unified python based build script for PhantomJS. On one hand, this merges the four build scripts (build.sh, build.cmd and preconfig.sh, preconfig.cmd) and thereby again decreases maintenance cost. Furthermore, it adds some magic to ensure e.g. that the git submodules are properly initialized. There are some more helpers in there, such as the ability to git clean qtbase/qtwebkit directories, see `build.py -h` for more information. Not all recent changes to the old build scripts are included in this new unified script, which is why I did not remove the old scripts yet.

https://github.com/ariya/phantomjs/issues/12426