Moving sources to src part 2: Adjust paths.

NOTE: To build after this you should rerun init-repository.py or run
$> git submodule sync
$> git submodule update
$> git config qtwebengine.chromiumsrcdir src/3rdparty

This makes everything build by adjusting paths.

Other mixed-in changes:
- Rename qtwebengine_src variables in scripts to qtwebengine_root to
  avoid confusion.
- Cleanup the release and debug extra targets that were in lib.pro.
  This file has also been split into src.pro and core.pro.

Change-Id: Ieee9158a65f526b15244eaca59e779b7069d337e
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>