getSite().getShell() is discouraged way to retrieve a Shell instance,
and check if the shell is disposed is wrong anyway for an editor which
can be closed but the shell is still there.

Added "disposed" flag and changed the checks accordingly to check if
*editor* is disposed.

Change-Id: I7bd015e5bfca1cfdeb76bf23784c2fd6a8efd666
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>