The WorkbenchActivitySupport is currently disabled quite late during
workbench shutdown. In particular, it is performed after the service
locator disposal, which produces several events for the disablement of
expression-controlled activities that are processed by the activity
management. Since the activity management is shut down afterwards
anyway, the processing of these events is unnecessary.

This change ensures that activities enablement changes are not
unnecessarily processed during workbench shutdown. It consists of two
parts:
1. It performs the disposal of activity management and its persistence
handler early in the workbench shutdown process (in particular before
the service locator disposal).
2. It adds proper dispose functionality for the IActivityManager, such
that disposing the workbench activity support disposes the activity
manager, which then removes all its listeners to not react to further
events.

Contributes to
https://github.com/eclipse-platform/eclipse.platform.ui/issues/1084.