GenericProject: Fix update on editing project.files

GenericProject: Fix update on editing project.files

The passed in filename is empty if the file is saved
with it current name. In that case pass the current
name to the GenericProjectManager, which then informs
the projects.

Change-Id: Id11c474930153e904702e7a0145e1f44331241fd
Reviewed-on: http://codereview.qt.nokia.com/1077
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>