API: Make the ability to update project files optional.

This is the functionality that allows callers to add files to groups
etc. Unfortunately, the QML/JS rewriter we use for this purpose pulls in
the Qt GUI module, which results in unnecessary library dependencies if
one only wants to use the command-line frontend. Therefore, allow this
feature to be compiled out.

Task-number: QBS-565
Change-Id: Ic134ff9cf37bd552ad493b388c05bcece983fd65
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>