Deploy QtWebEngineCore as a Qt module

This delegates the linking step from ninja to qmake so that we can
let qmake decide the destination of the target itself, easing the
deployment and installation logic across platforms.

The module is only deployed as a binary and no header are available
outside of the source tree. This is only to make sure that the
dependence of the QtWebEngine and QtWebEngineWidgets libraries on it
is resolved at runtime exactly the same way as with other Qt modules,
on all platforms.

Ninja still takes care of the compilation and gyp lets qmake know how
and what to link by dumping the list of flags and input files in a
generated .pri file.

This has to be done in a separate .pro file so that we can make sure
that ninja is run inconditionally before make reaches the dependency
check in core_module.pro, ensured by the parent Makefile.

Note 1: This patch removes RPATH hacks that are no longer necessary

Note 2: Other targets like ffmpegsumo are still linked by ninja. The
same logic co...