app: work around GTK bug causing UI freeze during tree-view operations
In GimpContainerTreeView, work around a GTK bug which can cause the window containing the tree view to stop processing updates, resulting in an apprarent freeze, until update-processing is explicitly requested. This can be triggered by adding layers to a layer group until the layers-dialog tree view is almost full, but still doesn't have a scrollbar, and then duplicating the group. This doesn't seem to affect GTK3, so not applying to master.
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