Previously configure was generating two config headers
qtwebengine-config.h and qtwebengine-config_p.h, however
those headers were never installed or included as dependency
in Makefiles. Moreover, due to the name clash all features
were included into qt_lib_webengine_*.pri which is
QtWebEngine QML module.

Move configure to core so all features belong now to
qt_lib_webenginecore*.pri. Fix global includes to include
qtwebenginecore-config*.h.

Drop all DEFINES and use QT_CONFIG instead.

Cleanup all evil looking includes in headers for webengine and
webenginewidgets.

Change-Id: Iddbc8bf4487d9a5f0c19a71a9569535083507756
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>