Add possibility to build without widgets or qml support.
Move module related options to webengine's subconfigure.

Make proper dependencies between qml_module and qml_plugins.
Cleanup headers.

Fixes: QTBUG-68956
Task-number: QTBUG-70784
Change-Id: I4605d98b0d2c83c99af37e2186b5fbf10f8a6049
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>