Fix position of indicator while tweening

Fix position of indicator while tweening

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