When changing the current theme via the Appearance preference page, and
disabling theming, two calls to IWorkbench.restart() are triggered
during ViewsPreferencePage.performOk(). This results in a hang at
shutdown, during the second restart call.

This change ensures only one restart is triggered, preventing the hang.

Fixes: #2950
Signed-off-by: Simeon Andreev <simeon.danailov.andreev@gmail.com>