QtWebKit build system cleanup

The idea is to clean up and improve how we use Qmake to build QtWebKit.

Remove Symbian support

Remove QTDIR_build and package-build

Add top level (root) pro-file for organizing the project (for QtCreator)

Use subdirs for derived sources instead of relying on build-webkit

Use CONFIG+=GNUMake when present to improve dependency tracking

TODO

• Move SBOX_CHECK to helpers.pri
• Move qt5-helper to helpers.pri