The spy part descriptors are marked as not to be persisted. If the user
opens this part as part via the show view menu, the generated
placeholder does not get the persistState flag and gets persisted. As
the part descriptor is removed, this reference is not valid and causes
the workbench model to be invalid and hence the state of the workbench
is restore.

This change persists also the part descriptor so that the reference
remains valid.

Change-Id: I219f9c5f5e739b760aada4c7cd15428649f1b2aa
Signed-off-by: Lars Vogel <Lars.Vogel@vogella.com>
Reviewed-on: https://git.eclipse.org/r/c/pde/eclipse.pde.ui/+/188092
Tested-by: PDE Bot <pde-bot@eclipse.org>