Make built-in JavaScript extensions available on demand.

This entails the following:
    - Project files can get access to built-in extensions in all
contexts via import statements such as "import qbs.TextFile".
    - In turn, the automatic injection of these extensions in rules and
probes no longer happens, i.e. one always has to explicitly import the
required extension.

Change-Id: Ib1e42c078354c564e417f80dd47897c7f41e7569
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>