D7560: Properly show/hide preview dialog

[Christian Ehrlicher]

Am 27.08.2017 um 16:46 schrieb Luigi Toscano:
>
> It works as expected. Technically the issue is not a regression, I've 
> just noticed also in the kdelibs4 version. It *may* be fixed there, 
> but up to you.
>
I can create a patch for master if someone is interested. But it may 
take some time since opensuse leap 42.3 does not seem to ship kdelibs4 
devel packages anymore...


Christian