getNamespaceIdentifier in Platform UI"

This reverts commit 9b3327873dfcf216d52b02bd1d3b4c958750a59f.

The right replacement for IExtension.getNamespace() is not
IExtension.getNamespace() but IExtension.getContributor().getName().

Change-Id: I253bfa0227bdebcae59d0a5935b9dbe6f0c55a4d
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>