Specify a fallback icon to QIcon::fromTheme()

The default fallback of QIcon::fromTheme() is QIcon(), i.e. a null icon.
Set the generic "unknown" icon as fallback to prevent missing icons for
filetypes that specify an icon name that doesn't exist.

This also gets rid of "QPixmap::scaled: Pixmap is a null pixmap"
warnings in that case.

BUG: 358958
BUG: 361034
FIXED-IN: 16.04.1
REVIEW: 127713