owimageviewer: Avoid RuntimeErrors after clearing the scene
owimageviewer: Avoid RuntimeErrors after clearing the scene

Do not use (Qt's) queued connection to set the retrieved image.
When a scene is cleared all the pending tasks are cancelled and target
widgets deleted, however if the signal is already enqueued, when the
scene is cleared, the signal handler would atempt to set the pixmap on
an already deleted widget.

Fixes gh-1316

cheral/orange3,qPCR4vir/orange3,cheral/orange3,cheral/orange3,cheral/orange3,qPCR4vir/orange3,qPCR4vir/orange3,cheral/orange3,qPCR4vir/orange3,qPCR4vir/orange3,cheral/orange3,qPCR4vir/orange3