This change exposes WorkbenchWindow.isClosing as API of
IWorkbenchWindow. Clients of WorkbenchPage can then check if the active
workbench window is closing, allowing problems such as bug 535462 to be
avoided.

Change-Id: I1629adf0b48ac17faa6ceb6f1dc1fe63350f0f8a
Signed-off-by: Simeon Andreev <simeon.danailov.andreev@gmail.com>