Because the overall parent allocation width immediately when the workspace
count changes, we were sometimes drawing the indicator in the wrong place
in the indicator animation that proceeded the remove-workspace animation.
Fix this by tweening only the Y position of the indicator and computing
the X position and size in our allocate() method. This also is considerably
simpler than switching the indicator between fixed position and geometry
managed.
https://bugzilla.gnome.org/show_bug.cgi?id=641881
53516733f7