The ActivityPersistenceHelper persists all activity enablements upon
every notification about a changed activity enablement it receives.
Persistence is, however, only done and thus necessary for activities
that are not expression-controlled. So the current implementation
performs unnecessary persistence operations.

With this change, the ActivityPersistenceHelper only persists the
enablements when the enablement of a non-expression-controlled activity
has changed. To this end, the received ActivityManagerEvent is extended
by the information about whether such an activity was affected. This
information is either directly passed to the event construction in case
it has already been calculated in the calling context or is calculated
lazily when accessing the value.

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