QmlProjectManager: Make sure project tree updates after in project file change

Use FileManager instead of our own FileSystemWatcher, since this one only signals
the first file change (cause Qt Creator apparently removes the file and creates
a new one on each change).

Change-Id: Idacd545e41cd3ee29bfbc122526b9097039c65d0
Reviewed-on: http://codereview.qt-project.org/4975


Reviewed-by: Daniel Teske <daniel.teske@nokia.com>
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>