[gui-dev] do not sort by priority if queue panel is invisible

[Jens-Uwe Mager]

Due to limited screen real estate I have on my computer the queue panel
invisible. Also with the type of content I am looking for I do not need
queueing, I am happy if one of the pending downloads I have completes
within a week or even longer. I was wondering recently why my queue table
does some fancy jumping, and I found that the queue is sorted by
priority even if the queue panel is not visible. The attached patch
changes that.