When a PartSashContainer is added as new root, it adopts the Composite
of the previous root. The Composite can be bound to the limbo-shell.
After which the new root is invisible.

Fix: ensure that the correct parent Composite is set.

Change-Id: I7312e8544f84fb95d93eba60d9dc6c26b3a4cc6b
Signed-off-by: Rolf Theunissen <rolf.theunissen@gmail.com>