Add .pro files for example subdirectories

Instead of adding individual example projects into SUBDIRS from the
top-level examples.pro, add a .pro file for both /examples/webengine
and /examples/webenginewidgets that handles them.

This way, it's easier to e.g. build all webengine examples from
binary packages, as the top-level examples.pro is not packaged.

Task-number: QTBUG-57124
Change-Id: I8cf8ac7b13b86306b31bc43c79e3d8272193b1d3
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>