When a window is not visible we cannot capture its thumbnail. We used to
store the thumbnail when a window was visible so that we could reuse it
when minimized, but this can cause visual glitches.

Let the GError ownership on the async finish method call.

The gtk_tree_view_set_model() adds its own reference.

Iteration over a linked list should be done over a side control variable
and list head needs to be preserved for proper list free.