CallWindow: hide the preview while dragging it
CallWindow: hide the preview while dragging it

We show a clone of it moving through the stage, so there's no
point in showing the main preview at the same time.

https://bugzilla.gnome.org/show_bug.cgi?id=656573

Distrotech/telepathy-account-widgets,GNOME/telepathy-account-widgets,GNOME/telepathy-account-widgets,GNOME/telepathy-account-widgets,Distrotech/telepathy-account-widgets