Move all the process code in the core library and let the later simply
call its QtWebEngine::processMain exported function from its main.

This also allows us building QtWebEngineProcess directly with qmake
without going through gyp.

Change-Id: I8df36510d0bf14e313918bef807e2118f1ecadd5
Reviewed-by: Andras Becsi <andras.becsi@digia.com>