shutdown

- don't call internalGetDecorator() without checking isEnabled() first
to avoid unintended re-creation of disabled decorator
- change order of shutdown calls to make sure the scheduler shutdown
happens before the decorators are disposed

Change-Id: I79986e2141d3860b8c6fde7ac87e031351e88452
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/184293
Tested-by: Platform Bot <platform-bot@eclipse.org>