Port the file layer fixes to master

CCBUG:328906

This contains:

commit 8d1a936d
Author: Boudewijn Rempt <boud@valdyas.org>
Date:   Wed Dec 18 14:03:46 2013 +0100

    BUG:328906 Fix scaling of file layer

commit 0ff91ca0
Author: Boudewijn Rempt <boud@valdyas.org>
Date:   Wed Dec 18 13:48:30 2013 +0100

    Fix crash on loading a file layer

    CCBUG:328906

commit de1b6ccc
Author: Boudewijn Rempt <boud@valdyas.org>
Date:   Wed Dec 18 12:45:54 2013 +0100

    BUG:328906 Fix scaling method labels for file layer dialog